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THE  THOUGHT  IS  IN  THE  QUESTION  THE  INFORMATION  18  IN  THE  ANSWER 


riAWKINs 


ELKJRia  mt 


^NSWER^ 


ILLUSTRATIONS 


A    PROGRESSIVE    COURSE  OF  STUDY 

FOR  ENGINEERS.  ELECTRICIANS,  STUDENTS 

AND  THOSE  DESIRING  TO  ACQUIRE  A 

WORKING  KNOWLEDGE  OF 

aEaRiciTv  m  it5  melons 

A  PRACTICAL  TREATISE 

® 

HAWKINS^^ND  STAFF 
THEO-    AyDEL\&     C0^^72  FIFTH  AVE.  NEW  YORK. 


COPYRIGHTED,  1914. 

BY 

THEO.  AUDEL  &  CO., 
New  York. 


I'finted  in  the  United  States. 


TABLE  OF  CONTENTS;  GUIDE  NO.  3 


,ry~'" 


TABLE  OF  CONTENTS 
GUIDE  NO.  3. 


GALVANOMETERS 431  to  464 

Action    of    compass    needle  —  simple    galvanometer  — 

difference  between  galvanoscope  and  galvanometer  —  sen- 
sibility—  action  of  short  and  long  coil  galvanometers  — 
classes  of  galvanometer  —  astatic  galvanometer  —  tan- 
gent galvanometer  —  graduation  of  tangent  galvanometer 
scale  —  table  of  galvanometer  constants  —  mechanical  ex- 
planation of  tangent  law  —  sine  galvanometer  —  table  of 
natural  sines  and  tangents  — •  comparison  of  sine  and  tangent 
galvanometers  —  differential  galvanometer  — -  ballistic  gal- 
vanometer —  kick  —  damping  effect  —  use  of  mirrors  in 
galvanometers  —  lamp  and  scale  —  damping  —  D'Arson- 
val  galvanometer:  construction,  operation;  uses  —  gal- 
vanometer constant  or  figure  of  merit  —  shunts. 


TESTING  AND  TESTING  APPARATUS-  465  to  536 

Pressure  measurement  —  Clark  cell  —  Weston  cadmium 
cell  —  pressure  measurement  error  with  ordinary  voltmeter 

—  International  volt — -hydraulic  analogy  of  amperes — coul- 
ombs —  current    measurement  —  International    ampere 

—  voltameters  —  Ohm's  law  and  the  ohm  —  International 
ohm  —  ohm  table  — •  practical  standards  of  resistance  — 
various  methods  of  resistance  measurement^ — direct 
deflection  method  —  method  of  substitution  —  resistance 
box  —  fall  of  potential  method  —  differential  galvanometer 
method  — ■  drop  method  • —  voltmeter  method  ■ —  Wheat- 
stone  bridge  —  usual  arrangement  of  resistances  of  Wheat- 
stone  bridge —  ratio  coils  of  Wheatstone  bridge  —  the  de- 
cade plan  —  two  plug  arrangement  —  "plug  out"  and 
"plug  in"  type  of  resistance  box  —  testing  sets  — direct 
deflection  method  with  Queen  Acme  set  —  ohmeter  —  fall 

7/^7 


TABLE  OF  CONTENTS;  GUIDE  NO.  3 


TESTING  AND  TESTING  AVVXKXTY?,— Continued. 

of  potential  method  with  Queen  Acme  set  —  apparatus 
for  measuring  low  resistances  —  how  to  check  a  volt- 
meter —  Kelvin  bridge  —  Queen  slide  wire  bridge  —  in- 
ternal resistance  measurement  —  Evershed  portable  ohm- 
meter  set  —  L  and  N  fault  finder  —  ammeter  test  —  dia- 
gram of  Queen  standard  potentiometer  —  diagrams 
illustrating  loop  testing  —  the  Murray  loop  —  the 
Varley  loop  —  special  loop  —  the  potentiometer  —  location 
of  opens  —  to  pick  out  faulty  wires  in  a  cable  —  voltage  of 
cell  measurement  with  potentiometer  —  care  of  potentio- 
meter —  location  of  faults  where  the  loop  is  composed  of 
cables  of  different  cross  sections. 


AMMETERS,    VOLTMETERS,    AND 

WATTMETERS 537  to  572 

Definition  of  ammeter  —  classification  of  ammeter  and 

voltmeters  —  moving  iron  type  instrument  —  Keystone 
voltmeter  —  winding  in  ammeters  and  volts  —  connections 
for  series  and  shunt  ammeters  —  voltmeter  connections 
—  Westinghouse  ammeter  shunts  —  various  types  of  in- 
strument—  plunger  type  instnmient  —  magnetic  vane  in- 
stnmient  —  inclined  coil  instniment  —  Whitney  hot  wire 
instruments  —  principle  of  electrostatic  instruments  — 
multipliers  —  portable  shunts  —  Siemens  electro-dyna- 
mometer —  station  instruments  —  Thompson  watt  hour 
meter  —  how  to  read  a  meter  —  installation  of  watt- 
meters—  Westinghouse  watt  hour  meter  —  Thompson 
prepa^-ment  watt  hour  meter  —  how  to  test  a  meter  — 
Sangamo  watt  hour  meter  —  Coliunbia  watt  hour  meter  — 
Duncan  watt  hour  meter. 


OPERATION  OF  DYNAMOS    -     -     -     -  573  to  596 

Before  starting  a  dynamo  —  adjusting  the  brushes  — 
brush  position  —  how  to  set  the  brushes  —  method  of 
soldering  cable  to  carbon  brush  —  brush  contact  pressure  — 
direction  of  rotation  —  method  of  winding  cables  with 
marlin  —  method  of  assembling  core  discs  —  starting  a 
dynamo  —  tinning  block  for  electric  soldering  tool  — 
shunt  dynamos  in  parallel  —  shunt  dynamos  on  three  wire 
system  —  how  to  start  a  series  machine  —  the  term  "build 
up" — how  to  start  a  shunt  or  compound  machine — "pick- 
ing up" — indication  of  reversed  connections  —  how  to 


TABLE  OF  CONTENTS;  GUIDE  NO.  S 


OPERATION  OF  DYNAMOS— Continued. 

correct  reversed  polarity  —  finding  the  reversed  coil  —  loss 
of  residual  magnetism  —  remedy  for  reversed  dynamo  — 
attention  while  running  —  lead  of  brushes  —  method  of 
taking  temperature  —  lubrication  —  oils  —  allowable  de- 
gree of  heating  —  attention  to  brushes  and  brush  gear. 


COUPLING  OF  DYNAMOS       -    -    -    -  597  to  610 

Series  and  parallel  connections  —  coupling  series  dynamos 
in  series;  in  parallel  —  equalizer  —  shunt  dynamos  in 
series;  in  parallel  —  switching  dynamo  into  and  out  of 
parallel  —  to  cut  out  a  machine  —  dividing  the  lead  — 
compound  dynamos  in  series;  in  parallel  —  equalizer 
connection  —  switching  a  compound  djmamo  into  and 
out  of  parallel  —  equalizing  the  load  —  shunt  and  com- 
pound dynamos  in  parallel. 


DYNAMO  FAILS  TO  EXCITE      -     -    -611  to  622 

Various  causes  —  brushes  not  properly  adjusted  —  de- 
fective contacts  —  incorrect  adjustment  of  regulators  — 
speed  too  low  —  testing  for  break  —  insufficient  residual 
magnetism;  remedy  —  open  circuits  —  test  for  field 
circuit  breakers  —  probable  location  of  breaks  —  Watson 
armature  discs  —  Fort  Wayne  commutator  truing  device — 
short  circuits  —  Watson  armature  —  wrong  connec- 
tions —  reversed  field  magnetism. 


ARMATURE  TROUBLES 623  to  634 

Causes  —  how  avoided  —  various  faults  —  short  circuit  in 
individual  coils  —  location  of  faulty  coil  —  test  for  break 
in  armature  lead  —  bar  to  bar  test  for  open  or  short  circuit 
in  coil  or  between  segments  —  short  circuits  between  ad- 
jacent coils  —  alternate  bar  test  for  short  circuits  between 
sections —  short  circuits  between  sections  through  frame  or 
core  of  armature;  between  sections  through  binding  wires — 
partial  short  circuits  in  armatures  —  method  of  testing 
for  breaks  —  burning  of  armature  coils  —  Watson  field 
coils  —  grounds  in  armatures  —  method  of  locating 
grounded  armature  coil  —  magneto  test  for  grounded  arma- 
tures —  method  of  binding  armature  winding  — 
breaks  in  armature  circuit. 


TABLE  OF  CONTENTS;  GUIDE  NO.  S 


CARE    OF    THE    COMMUTATOR    AND 

BRUSHES -     -  635  to  652 

Conditions  for  satisfactory  operation  —  oil  for  com- 
mutator —  attention  to  brushes  —  Bissell  brush  gear  — 
two  kinds  of  sparking  —  commutator  clamp  —  causes  of 
sparking  —  bad  adjustment  of  brushes  —  rocking  —  bad 
condition  of  brushes  —  brushes  making  bad  contact  —  bad 
condition  of  commutator  —  detection  of  untrue  commu- 
tator —  high  segments  —  "flats  " —  causes  of  flats;  remedy 
—  method  of  repairing  broken  joint  between  commutator 
segment  and  lug  —  segments  loose  or  knocked  in  —  how  to 
re-turn  a  commutator — Bissell  commutators  —  over- 
load of  dynamo  —  method  of  repairing  large  hole  burned  in 
two  adjacent  bars  of  a  commutator  —  operating  dynamos 
with  metal  brushes  —  indication  of  excessive  voltage  — 
method  of  smoothing  commutator  with  a  stone  —  causes 
of  excessive  voltage  —  loose  connections,  terminals,  etc., — 
breaks  in  armature  circuit  —  sandpaper  holder  for  com- 
mutator—short circuits,  in  armature  circuits;  infield  — 
breaks  in  field  —  sandpaper  block  —  short  circuits  in 
commutator. 


HEATING 653  to  662 

Various  causes  —  how  detected  —  procedure  —  heating, 
of  connections;  of  brushes,  commutator  and  armature  — 
excessive  heating  —  ventilated  commutator  —  self -oiling 
bearing  —  some  causes  of  hot  bearing  —  eff'ect  of  hot  bear- 
ings —  points  relating  to  hot  bearings  —  operation 
above  rated  voltage  and  below  normal  speed  —  forced 
system  of  lubrication  —  heating  of  field  magnets  — 
causes  of  eddy  currents  in  pole  pieces  —  detection  of  mois- 
ture in  field  coils  —  indication  of  short  circuits  in  field  coils. 


OPERATION  OF  MOTORS 663  to  696 

Before  starting  a  motor  —  starting  a  motor  —  various 

starting  resistances  —  starting  boxes  —  speed  regulators  — 
Cutler  Hammer  starter  —  tinie  required  to  start  motor 

—  how  to  start  —  sliding  contact  starters  —  series  motors 
on  battery  circuits  —  starting  a  shunt  motor  —  multiple 
switch  starters  —  eff'ect  of  reverse  voltage  —  rheostat 
with  no  voltage  and  overload  release  —  failure  to  start 

—  starting  panel  —  Cutler  Hammer  starting  rheostats  — 
Allen  Bradley  automatic  starter  —  Monitor  starter  with 


TABLE  OF  CONTENTS;  GUIDE  NO.  ."? 


OPERATION  OF  MOTORS— Continued. 

relay  for  push  button  control  —  a  remote  control  of  shunt 
motors  —  regulation  of  motor  speed ;  various  methods 

—  Monitor  printing  press  controller  —  speed  regulation  of 
series  motor,  by  short  circuiting  sections  of  the  field  winding 

—  varying  the  speed  of  shunt  and  compound  motors — Cutler 
Hammer  multiple  switch  starter  —  regulation  by  armature 
resistance  —  Compound  starter  —  regulation  by  shunt  field 
resistance  —  Holzer  Cabot  instructions  for  shunt  wound 
motor  —  Reliance  adjustable  speed  motor  —  Cutler  Ham- 
mer reversible  starter  —  combined  armature  and  shunt 
field  control  —  selection  of  starters  and  regulators  — 
Watson  commutators  —  organ  blower  speed  regulator  — 
General  Electric  controller  —  speed  regulation  of  traction 
motors  —  controller  of  the  Ranch  and  Lang  electric  vehicles 
• —  two  motor  regulation  —  controller  connection  dia- 
grams —  stopping  a  motor. 


t\y.^.    '%    W. 


GA  L  VA  NOME  TERS 


431 


CHAPTER  XXVI 
GALVANOMETERS 


If  a  compass  needle  be  allowed  to  come  to  rest  in  its  natural 
position,  and  a  current  of  electricity  be  passed  through  a  wire 
just  over  it  from  north  to  south,  the  north  seeking  end  of  the 
needle  will  be  deflected  toward  the  east.  If  the  wire  be  placed 
under  the  needle  and  the  current  continued  from  north  to  south 


TTdTT^ 


Fig.  503. — Effect  of  neighboring  current  upon  a  magnetic  needle.  Above  the  needle  and 
parallel  to  it  is  a  conductor  carrying  an  electric  current,  the  current  flowing  in  the  direction 
indicated  by  the  arrow.  This  causes  the  north  pole  of  the  needle  to  turn  toward  the  east. 
If  the  conductor  be  held  below  the  needle,  its  north  pole  will  turn  in  the  opposite  direction 
or  toward  the  west.  These  movements  are  easily  detemiined  by  Ampere's  rule  as  follows: 
//  a  man  could  swim  in  the  conductor  with  the  current,  and  turn  to  face  the  needle,  then  the 
north  pole  of  the  needle  will  be  deflected  toward  his  left  hand. 

the  needle  will  be  deflected  toward  the  west.  Again,  if  the  ctu*- 
rent  be  passed  from  north  to  south  over  the  needle,  and  back 
from  south  to  north  under  the  needle,  as  shown  in  fig.  504,  the 
magnetic  effect  will  be  doubled,  and  the  needle  deflected  pro- 
portionately. Upon  these  phenomena  depend  the  working  of 
galvanometers.  UtOKItTT  UMAST 


432  HAWKINS  ELECTRICITY 

Oues.     Describe  a  simple  galvanometer. 

Ans.  It  consists  essentially  of  a  magnetic  needle  suspended 
within  a  coil  of  vdve,  and  free  to  swing  over  the  face  of  a  graduated 
dial. 

Oues.  What  is  a  galvanoscope  and  how  does  it  diflfei 
from  a  galvanometer.' 

Ans.  A  galvanoscope,  as  shown  in  fig.  504,  serves  mereh'  lo 
indicate  the  presence  of  an  electric  current  without  measuring 


-^ 


Fig.  "MM. — Effect  upon  a  magnetic  needle  of  a  neighboring  current  in  a  loop.  In  this  arrange- 
ment the  same  conductor  is  simply  carried  back  beneath  the  needle  and  hence  both  the 
upper  and  lower  portions  tend  to  turn  it  in  the  same  direction,  while  the  side  branch  or 
vertical  section  is  ineffective.  In  accordance  wnth  Ampere's  swimming  rule,  the  upper 
wire  causes  the  N  pole  of  the  needle  to  turn  to  the  left,  while  if  a  man  imagine  himsel' 
swimming  in  the  lower  wire  in  the  direction  of  the  current,  and  facing  the  needle  (that  is, 
swimming  on  his  back),  the  N  pole  of  the  needle  will  turn  to  his  left — that  is  to  the  east. 
The  effect  of  the  loop  then  has  double  the  effect  of  the  single  wire  in  fig.  503. 


its  strength.  It  is  an  indicator  of  currents  where  the  movement 
of  the  needle  shows  the  direction  of  the  current,  and  indicates 
whether  it  is  a  strong  or  a  weak  one.  When  the  value  of  the 
readings  has  been  determined  by  experiment  or  calculation  any 
galvanoscope  becomes  a  galvanometer. 


GALVANOMETERS 


4.i:i 


Oues.     For  what  use  are  galvanometers  employed? 

Ans.  They  are  used  for  detecting  the  presence  of  an  electric 
current,  and  for  determining  its  direction  and  strength. 

Oues.  How  is  the  direction  and  strength  of  the  current 
indicated? 

Ans.  When  a  galvanometer  is  connected  in  a  circuit,  the 
direction  of  the  current  is  indicated  by  the  side  towards  which 
the  north  pole  of  the  needle  moves,  and  the  current  strength  by 
the  extent  of  the  needle's  deflection. 


Fig.  .")0o. — Effect  upon  a  magnetic  needle  of  a  neighboring  current  in  a  coil.  The  coil  as  shown, 
is  equivalent  to  several  loops,  that  is,  the  force  tending  to  deflect  the  needle  is  equal  to 
that  of  a  single  loop  multiplied  by  the  number  of  turns.  Hence,  by  using  a  coil  with  a 
large  number  of  turns,  a  galvanometer  may  be  made  very  sensitive  so  that  the  needle  will 
be  perceptibly  deflected  by  very  feeble  currents.  An  instrument,  as  shown  in  the  figure 
is  called  a  galvanoscope.  When  it  is  accurately  constructed,  and  supplied  with  a  scale 
showing  how  many  degrees  the  needle  is  deflected  it  is  then  called  a  galvanometer. 


Oues.     How  should  a  galvanometer  be  set  up  before 
using? 

Ans.     When  no  current  is  flowing,  the  coil  should  be  parallel 
to  the  magnetic  needle  when  at  rest. 


434  HAWKINS  ELECTRICITY 

Ques.     What  is  a  "  sensitive  "  galvanometer? 

Ans.     One  which  reqviires  a  very  small  current  or  pressure  to 
produce  a  stated  deflection. 

It  does  not  follow  that  a  galvanometer  which  is  sensitive  for  current 

measurement  will  also  be  sensitive  for  pressure  measurement. 

Ques.     Define  the  term  "  sensibility." 

Ans.     With  reference  to  mirror  reflecting  galvanometers  it 
may  be  defined  in  three  ways.    First,  in  megohms,  the  sensibility 


Fig.  506. — Bunnell  simple  detector  galvanometer.      It  has  middle  clamps  and  scale  divided 
into  degrees. 

being  the  number  of  megohms  through  which  one  volt  will  pro- 
duce a  deflection  of  one  millimeter  with  the  scale  at  one  meter 
distance.  Second,  in  micro-volts,  the  sensibiHty  being  the  number 
of  micro-volts  which  applied  directly  to  the  terminals  of  the 
galvanometer  will  produce  a  deflection  of  one  millimeter  on  a 
scale  one  meter  from  mirror.  The  sensibiHty  is  best  stated  in 
megohms  for  high  resistance  galvanometers  and  in  micro-volts 
for  low  resistance  galvanometers,  and  is  frequently  given  both 
for    galvanometers    for    intermediate    resistance.       Third,    in 


GALVANOMETERS 


435 


micro-amperes,  the  sensibility  being  the  number  of  micro- 
amperes that  will  give  one  millimeter  deflection  with  scale  at  a 
distance  of  one  meter. 

Ques.     Upon  what  does  the  sensibility  depend? 

Ans.  1,  Upon  the  number  of  times  the  current  circulates 
around  the  coil,  2,  the  distance  of  the  needle  from  the  coil,  3, 
the  weight  of  the  needle,  4,  the  current  strength,  and  5,  the 
amount  of  friction  produced  by  its  movement. 


Fig.    507.  —  Breguet    upright    galva- 
nometer with  glass  shade. 


Fig.  508. — Bunnell  horizontal  galvanometer.  It 
has  two  coils,  one  of  which  is  of  zero  resistance 
and  one  of  fifty  ohms  resistance  adapting  it  to  a 
variety  of  test. 


The  needle  is  usually  quite  small,  and  often  a  compound  one.  In  very 
sensitive  galvanometers,  the  coils  are  wound  with  thousands  of  turns  of 
very  fine  wire,  and  shunts  are  generally  used  in  connection  with  them. 


Ques.     What  two  kinds  of  coil  are  used  ? 

Ans.     The  short  coil  and  the  long  coil. 


NOTE. — Strong  currents  must  not  be  passed  through  very  sensitive  galvanometers,  for 
even  if  they  be  not  ruined,  the  deflections  of  the  needle  will  be  too  large  to  give  accurate  meas- 
urements. In  such  cases  the  galvanometer  is  used  with  a  shunt,  or  coil  of  wire  arranged  so 
that  the  greater  part  of  the  current  will  flow  through  it,  and  only  a  small  portion  through  the 
galvanometer. 


436 


HAWKINS  ELECTRICITY 


Oues.  What  is  the  difference  between  a  short  coil  and 
a  long  coil  galvanometer? 

Ans.  A  short  coil  galvanometer  has  a  coil  consisting  of  a  few 
turns  of  heavy  wire;  a  long  coil  galvanometer  is  wound  with  a 
large  number  of  turns  of  fine  wire. 


Fig.    !)09. — Bunnell  galvanometer  for  measurements  of  instruments,  lines,  batteries,  wires 
and  any  object  from  lij  to  10,000  ohms  or  more. 


Oues.  What  is  the  action  of  short  and  long  coil  gal- 
vanometers? 

Ans.  With  a  given  current,  the  total  magnetizing  force  which 
deflects  the  needle  is  the  same,  but  with  a  short  coil,  it  is  produced 
by  a  large  current  circulating  around  a  few  turns,  instead  of  a 
small  current  circulating  around  thousands  of  turns  as  in  the 
long  coil.  The  short  coil  being  of  low  resistance  is  used  to 
measure  the  current,  and  the  long  coil  with  high  resistance,  is 
suitable  for  measuring  the  pressure.  Hence,  a  short  coil  instru- 
ment with  its  scale  directly  graduated  in  ami^eres  is  an  ammeter, 
and  the  long  coil  type  with  graduation  in  volts  is  a  voltmeter. 


GA  L  VA  NO  METERS 


437 


Classes  of  Galvanometer. — ^There  are  numerous  kinds  of 
galvanometer  designed  to  meet  the  varied  requirements. 
According  to  construction,  galvanometers  may  be  divided  into 
two  classes,  as  those  having  : 

1.  Movable  magnet  and  stationary  coil; 

2.  Stationary  magnet  and  movable  coil. 


Pig.  510. — Astatic  needles.  Two  magnetic  needles  of  equal  moment  are  mounted  in  opposition 
on  a  light  support.  The  whole  system  is  suspended  by  a  delicate  fibre,  and  when  placed 
in  a  uniform  magnetic  field  such  as  that  of  the  earth,  there  will  be  no  tendency  to  assume 
any  fi.xed  direction,  the  only  restraining  influence  oil  the  needles  being  that  due  to  torsion 
in  the  suspension  fibre. 

Either  type  may  be  constructed  with  short  or  long  coil,  and 
there  are  several  ways  in  which  the  deflections  are  indicated. 
The  principal  forms  of  galvanometer  are  as  follows: 


1.  Astatic; 

2.  Tangent; 

3.  Sine; 

4.  Differential; 

5.  Ballistic; 

0.  D' Arson val. 


438 


HAWKINS  ELECTRTCTTV 


Astatic  Galvanometer. — It  has  been  pointed  out  how  a 
compass  needle  is  affected  when  a  wire  carrying  a  current  is  held 
over  or  under  it,  the  needle  being  turned  in  one  direction  in  the 
first  instance,  and  in  the  opposite  direction  for  the  second  posi- 
tion of  the  vnre. 

The  earth's  magnetism  naturally  holds  the  compass  needle 
north  and  south.  The  magnetic  field  encircling  the  wire,  being 
at  right  angles  to  the  needle  (when  the  wire  itself  is  parallel  there- 
with), operates  to  turn  it  from  its  normal  position,  north  and 
south,  so  as  to  set  it  partially  east  and  west.    However,  on  account 


Fig.  511. — Connections  of  single  coil  astatic  needles.     The  coil  surrounds  the  lower  needle 
and  the  direction  of  the  current  between  the  two  needles  tends  to  turn  them  the  same  way. 


of  the  fact  that  the  earth's  magnetism  does  exert  some  force 
tending  to  hold  the  needle  north  and  south,  it  is  evident  that  no 
matter  how  strong  the  current,  the  latter  can  never  succeed  in 
turning  the  needle  entirely  east  and  west.  The  accomphshment 
of  this  is  further  prevented  by  the  reason  of  the  points  of  the 
needle,  where  the  magnetic  effect  is  greatest,  quickl}'-  passing  out 
of  the  reach  of  the  magnetic  field,  where  it  is  now  practically 
operated  on  only  in  a  slight  degree.  Thus  it  would  take  quite  a 
powerful  current  to  hold  the  needle  deflected  any  appreciable 
distance.    The  use  of  a  shorter  needle  is.  therefore,  more  desirable. 


GALVANOMETERS 


439 


It  is  evident  in  this  style  of  instrument  that  the  effect  of  the 
current  cannot  be  accurately  measured,  because  it  acts  in  opposi- 
tion to  the  earth's  magnetism,  and  as  this  is  constantly  varying, 
some  method  must  be  employed  which  will  either  destroy  the 
earth's  magnetism  or  else  neutralize  it. 

In  the  astatic  galvanometer,  the  earth's  magnetism  is  neutral- 
ized by  means  of  astatic  needles.  These  consist  of  a  combination 
of  two  magnetic  needles  of  equal  size  and  strength,  connected 


S  «gs^p^^»|y]      ) 


HD 


Fig.  51*. — Connections  of  double  coil  astatic  needles.  With  this  arrangement,  the  direction 
of  current  in  both  coils  will  tend  to  turn  the  system  in  the  same  direction,  making  the 
needles  more  sensitive  than  with  a  single  coil  as  in  fig.  511. 


rigidly  together  with  their  poles  pointing  in  opposite  and  parallel 
direction?,  as  shown  in  fig,  510.  As  the  north  pole  of  the  earth 
attracts  the  south  pole  of  one  of  the  needles,  it  repels  with  equal 
strength  the  north  pole  of  the  other  needle,  hence,  the  combination 
is  independent  of  the  earth's  magnetism  and  will  remain  at  rest 
in  any  position. 

If  one  of  the  needles  be  surrounded  by  a  coil,  as  shown  in  fig. 
511,  the  magnetic  effect  of  the  current  will  be  correctly  indicated 
by  the  deflection  of  the  needle. 


440  HAWKINS  ELECTRICITY 

Sometimes  each  needle  is  surrounded  by  a  coil,  as  in  fig.  512, 
the  coils  being  so  connected  that  the  direction  of  current  in  each 
mil  tend  to  deflect  the  needles  in  the  same  direction, 

Oues.  For  what  use  is  the  astatic  galvanometer 
adapted? 

Ans.     For  the  detection  of  small  currents. 

It  is  used  in  the  "nil"  or  zero  methods,  in  which  the  current  between 
the  points  to  which  the  galvanometer  is  connected  is  reduced  to  zero. 


Fig.  513. — Queen  reflecting  astatic  galvanometer.  It  is  mounted  on  a  mahogany  base  with 
levelling  screws.  A  plain  mirror  is  attached  above  the  upper  needle.  The  entire  combina- 
tion of  mirror  and  needles  is  suspended  by  unspun  silk  from  the  interior  of  a  brass  tube, 
which  also  carries  a  weak  controlling  magnet.  A  dial  4  inches  in  diameter  and  graduated 
in  degrees,  enables  the  deflections  of  the  needle  to  be  accurately  read.  The  mirror  can  be 
used  with  a  reading  telescope  and  scale,  or  by  means  cf  a  lantern,  the  image  of  a  slit  may 
be  reflected  from  the  mirror  to  a  screen.    Resistance,  .5  to  1 ,000  ohms. 

Oues.     Upon  what  does  the  movement  of  the  needles 
depend? 

Ans.     Upon  the  combined  effect  of  the  magnetic  attraction  of 
the  current  which  tends  to  deflect  the  needles,  and  the  torsion 


GALVANOMETERS  441 


in  the  suspension  fibre  which  tends  to  keep  the  needle  at  the 
zero  position. 

Oues.  Does  the  astatic  galvanometer  give  correct 
readings  for  different  values  of  the  current? 

Ans.  When  the  deflections  are  small  (that  is,  less  than  10° 
or  15°),  they  are  very  nearly  proportional  to  the  strength  of  the 
currents  that  produce  them. 


Fig.  514. — Central  Scientific  Co.  tangent  galvanometer.  A  9  inch  brass  ring  is  mounted 
on  a  mahogany  base  which  rotates  on  a  tripod  provided  with  levelling  screws.  The  needle 
has  an  aluminum  pointer  and  jewelled  bearing.  The  winding  consists  of  300  turns  of  mag- 
net wire  so  connected  to  the  plugs  in  front  that  20,  40,  SO,  or  160  turns  ox  any  combination 
of  these  numbers  may  be  used.  For  heavy  currents  a  band  of  copper  is  used  by  connectiiig 
to  the  extra  pair  of  binding  posts  in  the  rear  of  the  instrument. 

Thus,  if  a  current  produce  a  deflection  of  6°  it  is  known  to  be  approxi- 
mately three  times  as  strong  as  a  current  which  only  turns  the  needle 
through  2°.  But  this  approximate  proportion  ceases  to  be  true  if  the 
deflection  be  more  than  15°  or  20°. 

Oues.     "Why  does   the   instrument   not  give  accurate 
readings  for  large  deflections? 

Ans.     The  needles  arc  not  so  advantageously  acted  upon  by 
the  current,  since  the  poles  are  no  longer  within  the  coils,  but 


442 


HAWKINS  ELECTRICITY 


protrude  at  the  side.  Moreover,  the  needles  being  oblique  to 
the  force  acting  on  them,  part  only  of  the  force  is  turning  them 
against  the  directive  force  of  the  fibre ;  the  other  part  is  uselessly 
piilling  or  pushing  them  along  their  length. 

Ques.     How  may  correct  readings  be  obtained? 

Ans.     The  instrument  may  be  calibrated,  that  is,  it  may  be 
ascertained  by  special  measurements,  or  by  comparison  with  a 


Fig.  515. — Bvinnell  tangent  galvanometer.  This  instrument  is  mounted  on  a  circular  hard 
rubber  base,  7?^  inches  diameter,  provided  with  levelling  screws  and  anchoring  points. 
The  galvanometer  consists  of  a  magnetized  needle  "s  inch  in  length,  suspended  at  the 
center  of  a  rubber  ring  six  inches  in  diameter,  containing  the  coils.  There  are  five  coils  of 
0,  1,  10,  50  and  150  ohms  resistance.  The  first  is  a  stout  copper  band  of  inappreciable 
resistance;  the  others  are  of  different  sized  copper  wires,  carefully  insulated.  Five  ter- 
minals are  provided,  marked,  respectively,  0,  1,  10,  50  and  150.  The  ends  of  the  coils  are 
so  arranged  that  the  plug  inserted  at  the  terminal  marked  50  puts  in  circuit  all  the  coils; 
marked  at  the  terminal  50 — all  e.xcept  the  150  ohm  coil;  and  so  on,  till  at  the  zero  terminal 
only  the  copper  band  is  in  circuit.  Fixed  to  the  needle,  which  is  balanced  on  jewel  and 
point,  is  an  aluminum  pointer  at  right  angles,  extending  across  a  five  inch  dial  immediately 
beneath.  One  side  of  the  dial  is  divided  into  degrees;  on  the  other  side,  the  graduatieas 
correspond  to  the  tangent  of  the  angles  of  deflectioa. 


GALVANOMETERS  443 


standard  instrument,  the  amounts  of  deflection  corresponding 
to  particular  current  strengths. 

Thus,  if  it  be  once  known  that  a  deflection  of  32°  on  a  particular 
galviinometer  is  produced  by  a  current  of  jj^  of  an  ampere,  then  a 
current  of  that  strength  will  always  produce  on  that  instrument  the 
same  deflection,  unless  from  any  accident  the  torsion  force  or  the  in- 
tensity of  the  magnetic  field  be  altered. 


Pig.  516. — Tangent  galvanometer.  It  consists  of  a  short  magnetic  needle  suspended  at  the 
center  of  a  coil  of  large  diameter  and  small  cross  section.  In  practice,  the  diameter  of  the 
coil  is  about  17  times  the  length  of  the  needle.  If  the  instrument  be  so  placed  that,  when 
there  is  no  current  in  the  coil,  the  suspended  magnet  lies  in  the  plane  of  the  coil,  that  is, 
if  the  plane  of  the  coil  be  set  in  the  magnetic  meridian,  then  the  current  passing  through  the 
coil  is  proportional  to  the  tangent  of  the  angle  by  which  the  magnet  is  deflected  from  the  plane 
of  the  coil,  or  zero  position — hence  the  name:  "tangent  galvanometer." 

The  Tangent  Galvanometer. — It  is  not  possible  to  construct 
a  galvanometer  in  which  the  angle  (as  measured  in  degrees  of  arc) 
through  which  the  needle  is  deflected  is  proportional  throughout 
its  whole  range  to  the  strength  of  the  current.  But  it  is  possible 
to  construct  a  very  simple  galvanometer  in  which  the  tangent  oj 
the  angle  oj  deflection  shall  be  accurately  proportional  to  the 
strength  of  the  current. 


444 


HAWKINS  ELECTRICITY 


A  simple  form  of  tangent  galvanometer  is  shown  in  fig.  516 
The  coil  of  this  instrument  consists  of  a  simple  circle  of  stout 
copper  wire  from  ten  to  fifteen  inches  in  diameter.    At  the  center 
is  delicately  suspended  a  magnetized  steel  needle  not  exceeding 


Fig.  517. — Horizontal  section  through  middle  of  tangent  galvanometer,  showing  magnetic 
whirls  around  the  coil  and  corresponding  deflection  of  needle. 

one  inch  in  length,  and  usually  furnished  with  a  light  index  of 
aluminum.  When  the  galvanometer  is  in  use,  the  plane  of  the 
ring  must  be  vertical  and  in  the  magnetic  meridian.  A  hori- 
zontal section  through  the  middle  of  the  instrument  is  shown  in 


V^' 


Fig.  518. — Diagram  of  forces  acting  on  the  needle  of  a  tangent  galvanometer. 

fig.  517.  For  simplicity,  the  coil  is  supposed  to  have  but  a  single 
turn  of  wire,  the  circles  surrounding  the  wire  representing  the 
magnetic  lines  of  force.  By  extending  the  lines  of  force  until 
they  reach  the  needle,  it  will  be  seen  that  with  a  short  needle,  the 


GALVANOMETERS 


445 


deflecting  force  acts  in  an  east  and  west  direction  when  the  gal- 
vanometer is  placed  with  its  coil  in  the  magnetic  meridian. 

If,  in  fig.  518,  ab  represent  the  deflecting  force  acting  en  the  N  end  of 
the  needle,  the  component  of  this  force  that  acts  at  a  right  angle  to  the 
needle  will  be 

ab  cos  X 

in  which,  x  is  the  angle  of  the  deflection. 
The  controlling  force  is 

aJ  =  H 


i 

I            1 

b              / 

i            i 

H 

( 

1 
R 

1 

I 

^ 

o^ 

. — 

M 

:^ 

< 

0 

>■ 

>■ 

Fig.  519. — Diagram  illustrating  the  tangent  law.  This  is  the  law  of  the  combined  action  of 
two  magnetic  fields  upon  a  magnetic  needle.  If  two  magnetic  fields  be  at  right  angles  in 
direction  as  indicated  in  the  figure,  the  resultant  field  is  obtained  by  the  parallelogram  of 
forces  and  it  makes  an  angle  0  with  one  of  the  component  fields  such  that  tan 
0  =  M  +  H  where  M  and  H  are  the  strengths  of  the  component  fields.  In  the  tangent 
galvanometer  this  principle  is  employed  in  the  measurement  of  currents.  A  magnetic  needle 
is  pivoted  in  a  field  of  known  strength.  The  current  to  be  measured  is  passed  round  a  coil 
(or  coils)  which  generates  a  field  at  risht  angles  to  the  original  field.  The  needle  then  lies 
along  the  direction  of  the  resultant  field,  and  by  finding  the  tangent  of  its  angle  of  deflection, 
and  knowing  the  field  strength  produced  by  unit  current  in  the  coil,  the  current  strength 
can  be  found. 


and  when  the  needle  is  in  equilibrium,  the  component  ae  =  H  sin  x  is 
equal  and  opposite  to  ac,  hence 


from  which 


ab  cos  «  =  H  sin  x 


ab  =  H =  H  tan  x 

cos  X 


Since  ab  is  proportional  to  the  current, 

ab  =  k  C  =  K  tan  x 


446 


HAWKINS  ELECTRICITY 


in  which  it  is  a  constant  depending  upon  the  instrument.    For  any  other 
current  C, 

kC  =  K  tan  x' 
hence 

C:  C  =  tanx  :  tan  a;' 

This  means  that  the  currents  passing  through  the  coil  of  a  tangent 
galvanometer  are  proportional,  not  to  the  angle  of  deflection,  but  to  the 
tangent  of  that  angle. 


30   T 


Fig.  520. — Graduation  of  tangent  galvanometer  scale  with  divisions  representing  tangent 
values.  In  the  figure  let  a  tangent  OT  be  drawn  to  the  circle,  and  along  this  line  let  any 
number  of  equal  di\-isions  be  set  off,  beginning  at  O.  From  these  points  draw  lines  back  to 
the  center.  The  circle  will  thus  be  divided  into  a  number  of  spaces,  of  which  those  near  O 
are  nearly  equal,  but  which  get  smaller  and  snialler  as  they  recede  from  O.  These  unequal 
spaces  correspond  to  equal  increments  of  the  tangent.  If  the  scale  were  divided  thus,  the 
readings  would  be  proportional  to  the  tangents. 


Oues.  Upon  what  does  the  sensibility  of  a  tangent 
galvanometer  depend? 

Ans.  It  is  directly  proportional  to  the  number  of  turns  of  the 
coil  and  inversely  proportional  to  the  diameter  of  the  coil. 

Oues.  How  may  the  tangent  galvanometer  be  used  as 
an  ammeter? 

Ans.  The  strength  of  the  current  may  be  calculated  in  am- 
peres by  the  formula  given  below  when  the  dimensions  of  the 
instrument  are  known. 

The  needle  is  supposed  to  be  subject  to  only  the  earth's  magnetism 
and  to  move  in  a  horizontal  plane.    The  current  is  calculated  as  follows: 


amperes  = 


H_x_ 

N 


tan  X. 


(1) 


GA  L  VA  NOME  TERS 


447 


in  which 

H  =  constant  from  table  below; 
r  =  radius  of  coil ; 
N  =  number  of  turns  of  coil ; 
X  =  angle  of  deflection  of  needle. 

The  constant  H,  given  in  the  following  table  represents  the  horizontal 
force  of  the  earth's  magnetism  for  the  place  where  the  galvanometer  is 

used.    Each  value  has  been  multiplied  by  -—  so  that  the  formula  (i)  for 

amperes  is  correct  as  given. 


Fig.  521. — Mechanical  explanation  of  the  tangent  law.  Construct  an  apparatus  as  shown  in  the 
figure.  The  short  wooden  block,  NS,  represents  the  magnetic  needle.  This  fiece  of  wood 
turns  around  its  center,  C,  which  may  be  an  ordinary  nail.  It  will  now  be  seen  that  two 
different  forces  act  upon  N;  namely,  the  weight.  G  (oneor  two  ounces),  and  the  changeable 
weights  which  are  placed  in  the  scoop,  W  (made  of  cardboard).  Tue  height  of  the  roll,  or 
wheel,  R,  is  such  that  the  cord,  RN,  runs  horizontally,  when  NSstands  vertically,  i.e.,  when 
there  is  no  weight  in  the  little  scoop.  If  the  wheel,  R,  be  placed  '-ufficienlly  far  from  NS, 
the  string  RN,  will  always  remain  almo.=t  horizontal,  even  if  NS  be  deviated.  The  thin 
hand  on  NS  moves  over  a  vertical  scale,  which  is  divided  into  equal  parts,  as  shown.  This 
scale  may  be  made  of  cardboard.  If  the  hand  point  to  division  1  when  one  ounce  is  placed 
in  the  scoop,  it  will  point  to  2  for  two  ounces,  to  3  for  three  ounces,  etc.  At  45°  the  needle 
is  debated  at  its  greatest  angle,  and  this  is,  therefore,  the  sensiti\^ty  angle  of  the  tangent 
galvanometer.  The  deviating  values  are,  therefore,  proportionate  to  the  scale  parts  01, 
02,  and  03,  and  so  on;  and,  inasmuch  as  these  themselves  are  tangents,  the  tangent  law 
will  hold  good. 


Table  of  Galvanometer  Constants. — Values  of  H. 


Boston 
Chicago  . 
Denver    . 
Jacksonville 
London    . 
Miimeapolis 
New  York 


.699 
.759 
.919 
1.094 
.745 
.681 
.744 


New  Haven 731 

Philadelphia 783 

Portland,  Me 674 

San  Francisco       ....      1.021 

St.  Louis 871 

Washington 810 


448  HAWKINS  ELECTRICITY 

Oues.  How  is  the  tangent  galvanometer  constructed 
to  give  direct  readings? 

Ans.  To  obviate  reference  to  a  table,  the  circular  scale  of 
the  instrument  is  sometimes  graduated  into  tangent  values,  as 
in  fig.  520,  instead  of  being  divided  into  equal  degrees. 


Pig.  522, — Queen  tangent  and  <«"«'  galvanometer.  This  instrument  properly  adjusted  can  be 
used  as  a  standard  instrument  for  laboratory  work.  The  brass  ring  is  12  inches  in  diam- 
eter, and  the  grooves  in  which  the  wire  is  wound  are  carefully  turned  so  as  to  be  of  true 
rectangular  cross  section,  thus  allowing  the  constant  of  the  instrument  to  be  accurately  cal- 
culated and  compared  with  the  constant  as  obtained  by  other  methods.  The  compass  box 
is  5  inches  in  diameter  and  is  so  held  in  position  that  it  may  be  raised  or  lowered,  rotated 
on  its  vertical  axL  shL'ted  out  of  the  plane  of  the  coil,  etc.,  thus  enabling  the  opverator  to 
acquire  proficiency  with  the  instrument  and  to  meet  all  cases  of  derangement  possible  The 
dial  is  graduated  to  single  degrees,  and  the  needle  is  suspended  by  a  very  light  cocoon  fibre. 
The  whole  instrument  can  be  turned  about  itr.  vertical  a.'us,  and  a  quadrant  graduated  in 
degrees  upon  the  base  allows  the  amount  of  rotation  to  be  accurately  measured,  and  the 
laws  of  the  sine  galvanometer  investigated.  The  instrument  is  wound  to  measure  .25 
ampere  to  8  amperes, 

Oues.     What  is  the  objection  to  the  scale  with  tangent 
values? 

Ans.     It  is  more  difficult  to  divide  an  arc  into  tangent  lines 
with  accuracy  than  into  equal  degrees. 

Oues.    What    disadvantage    has    the    tangent    galva- 
nometer? 

Ans.     The  coil  being  much  larger  than  the  needle,  and  hence 
far  away  from  it,  reduces  the  sensitiveness  of  the  instrument. 


GALVANOMETERS 


449 


The  Sine  Galvanometer. — This  type  of  instrument  has  a 
vertical  coil  which  may  be  rotated  around  a  vertical  axis,  so  that 
it  can  be  made  to  follow  the  magnetic  needle  in  its  deflections. 

In  the  sine  galvanometer,  the  ceil  ts  mov^ed  so  as  to  follow  the  needle 
until  it  is  parallel  with  the  coil.  Under  these  circumstances,  the  strength 
of  the  deflecting  current  is  proportional  to  sine  of  angle  of  deflection. 


Fig.  523. — Central  Scientific  Co.  universal  tangent  galvanometer.  This  instrument  may  be 
used  as  a  tangent,  Gaugain,  Helmholtz-Gaugain,  sine,  cosine,  Wiedemann  or  detector 
galvanometer.  The  coils,  which  slide  on  a  beam  parallel  to  the  one  carrying  the  needle  box, 
are  wound  on  brass  rings  12  inches  in  diameter.  On  each  ring  are  wound  two  coils  of  48 
turns  each,  c  nnected  to  separa.e  binding  posts,  and  double  wound  so  as  to  be  of  equal 
resistance.  The  coils  and  needle  bo.x  are  each  pro\'ided  with  an  indicator  for  reading  their 
position  on  the  scale.  The  needle  box  is  swivelled  and  removable  and  one  coil  may  be 
rotated  about  its  vertical  axis  and  its  position  read  on  a  disc  graduated  in  degrees.  Cur- 
rents may  be  measured  ranging  from  .000002  ampere  to  ICO  amperes. 


Oues.     Describe  the  construction  of  a  sine  galvanometer. 

Ans.  A  form  of  sine  galvanometer  is  shown  in  fig.  524.  The 
vertical  wire  coil  is  seen  at  M.  A  needle  of  any  length  less  than 
the  diameter  of  the  coil  M,  moves  over  the  graduated  circle  N. 
The  coil  M,  and  graduated  circle  N  may  be  rotated  on  a  vertical 


450 


HAWKINS  ELECTRICITY 


axis,  and  the  amount  of  angular  movement  necessary  to  bring 
the  needle  to  zero,  measured  on  the  graduated  circle  H. 

Oues.     How  is  the  current  strength  measured? 

Ans.  It  is  proportional  to  the  sine  of  the  angle  measured  on 
the  horizontal  circle  H,  through  which  it  is  necessary  to  turn  the 
coil  M,  from  the  plane  of  the  earth's  magnetic  meridian  to  the 
plane  of  the  needle  when  it  is  not  further  deflected  by  the  current. 


Fig.  524. — Sine  galvanometer.  It  differs  from  the  tangent  galvanometer  in  that  the  vertical 
coil  and  magnetic  needle  are  mounted  upon  a  standard  free  to  revolve  around  a  vertical 
axis,  with  provision  for  determining  the  angular  position  of  the  coil.  The  needle  may  be 
of  any  length  shorter  than  the  diameter  of  the  coil.  In  the  figure  the  parts  are:  M,  coil; 
N,  graduated  dial  of  magnetic  needle;  H,  graduated  dial  by  which  the  amount  of  rotation 
necessary  to  bring  the  needle  to  zero  is  measured;  E,  terminals  of  the  coil;  O,  upright 
standard  carrying  coil  and  graduated  dial  of  magnetic  needle;  C,  base  with  levelling  screws. 


Oues.     How  is  the  sine  galvanometer  operated? 

Ans.  In  using  the  instrument,  after  the  needle  has  been  set 
to  zero,  the  current  is  sent  through  the  coil,  producing  a  deflec- 
tion of  the  needle.    The  coil  is  then  rotated  to  follow  the  motion 


GALVANOMETERS 


451 


of  the  needle,  the  current  being  kept  constant,  the  rotation  being 
continued  until  the  zero  on  the  upper  dial  again  registers  with  the 
needle.  The  current  then  is  proportional  to  the  sine  of  the  angle 
through  which  the  coil  has  been  turned,  as  determined  by  the 
lower  dial. 

Oues.     Has  the  sine  galvanometer  a  large  range? 

Ans.  For  a  given  controlling  field,  it  does  not  admit  of  a  very 
large  range  of  current  measurement,  since,  for  large  deflection, 
on  rotating  the  coil  the  position  of  instability  is  soon  reached. 


TABLE 

OF 

NATURAL  SINES 

AND 

TANGENTS 

^ 

Sin. 

Tan. 

^ 

Sin. 

Tan. 

^ 

Sia. 

Tan. 

I 

Sin. 

Tan. 

^ 

Sin. 

Tan. 

0° 

.0000 

.0000 

18° 

.3090 

.3249 

36° 

.5878 

7265 

54° 

.8090 

1.3764 

72° 

.9511 

3.0777 

19 

.3256 

.3443 

55 

.8192 

1.4281 

73 

9563 

3.2709 

1 

.0175 

.0175 

37 

.6018 

.7536 

56 

.8290 

1.4826 

2 

.0349 

.0:^49 

20 

.3420 

,3640 

38 

.6157 

.7813 

74 

9613 

3.4874 

8 

.0523 

.0524 

39 

.6293 

8098 

57 

.8387 

1.5399 

75 

.9659 

3.7321 

21 

.3584 

.3839 

58 

.8480 

1.6003 

76 

.9703 

4.U1U8 

4 

.0698 

.0699 

22 

.3746 

.4040 

40 

.6428 

.8391 

59 

.8572 

1.6643 

6 

.0871 

.0875 

23 

.3907 

.4245 

77 

.9744 

4.3315 

6 

.1045 

.1051 

41 

.6.561 

.8693 

60 

.8660 

1.7321 

78 

.9781 

4.7040 

24 

.4067 

.4452 

42 

.6691 

.9004 

79 

9816 

5.1446 

7 

.1219 

.1228 

25 

.4226 

.40C.:f 

43 

.6820 

9325 

61 

.8746 

1.8040 

i 

.1392 

.1405 

26 

.4384 

.4877 

62 

.S829 

1.8807 

80 

.9848 

5.0713 

9 

.1564 

.1564 

44 

.6947 

.9657 

63 

.8910 

1.9626 

27 

.4540 

.509i 

45 

.7071 

1.0000 

81 

9877 

6.3138 

10 

.1736 

.1763 

28 

.4695 

.5317 

46 

.7193 

1.0355 

64 

.8988 

2.0503 

82 

.9913 

7.11.14 

29 

.4848 

.5543 

65 

9063 

2.1445 

83 

9925 

8.1443 

11 

.1908 

.1944 

47 

.7314 

1.0724 

66 

.9135 

2.24o0 

12 

.2079 

.2126 

30 

.5000 

.5774 

48 

.7431 

1.1106 

84 

.9945 

9.5144 

13 

.2250 

.2309 

49 

.7547 

1.1504 

57 

.9205 

2.35.59 

85 

.9962 

11.43 

31 

.5150 

.6009 

68 

.9272 

2.4751 

86 

.9976 

14.30 

14 

.2419 

.2493 

32 

.5299 

.6249 

50 

7660 

1.1918 

69 

.9339 

26051 

15 

.2588 

.2679 

33 

.5446 

.6494 

87 

.9986 

19.03 

16 

.2756 

.2867 

51 

.7771 

1.2349 

70 

.9397 

2.7175 

.S8 

.9994 

2-<.64 

34 

.5592 

.6745'52 

.7880 

1.2799 

S9 

.9998 

57.29 

17 

.2924 

.3057 

35 

.6736 

.7002^3 

.7986 

1.3270 

71 

.9455 

2.9042 

Oues.     What  is  the  position  of  instability? 

Ans.     The  position  of  the  needle  beyond  which  the  rotation  of 
the  coil  will  cause  it  to  turn  all  the  way  round. 

Oues.     How  may  the  range  be  increased? 

Ans.     By  an  adjustable  controlling  field  or  a  shunt. 


452  HAWKINS  ELECTRICITY' 

Oues.  What  advantage  has  the  sine  galvanometer  over 
the  tangent  instrument? 

Ans.  Its  advantage  is  in  the  case  where  the  relative  values 
of  two  or  more  currents  arc  required  to  be  measured,  or  where 
the  constant  of  the  instrument  is  obtained  by  comparison  with 
a  standard  measuring  instrument  and  not  calculated  from  the 
dimensions  of  the  coil,  because  all  galvanometers  thus  used  follow 
the  sine  law  independently  of  the  shape  of  the  coil,  while  only 
circular  coils  will  follow  the  sine  law. 


Pig.  525. — Differential  galvanometer.  It  consists  of  two  coils  of  wire,  so  wound  .is  to  have 
opposite  magnetic  effects  on  a  magnetic  needle  suspended  centrally  between  them.  The 
needle  of  a  differential  galvanometer  shows  no  deflection  when  two  equal  currents  are 
sent  through  the  coils  in  opposite  directions,  since,  under  these  conditions,  each  coil 
neutralizes  the  effect  of  the  other.  Sometimes  the  current  is  so  sent  through  the  two  coils, 
that  each  coil  deflects  the  needle  in  the  same  direction.  In  this  case  the  instrument  is  no 
longer  differential  in  action.  If,  when  this  condition  obtains,  the  magnetic  needle  be  sus- 
pended at  the  exact  center  of  the  line  which  joins  the  centers  of  the  coils,  the  advantage 
is  gained  by  obtaining  a  field  of  more  nearly  uniform  intensity  around  the  needle.  When 
the  needle  is  suspended  by  a  silk  fibre,  a  final  and  most  delicate  adjustment  can  be  obtained 
by  raising  or  lowering  one  of  the  levelling  screws  slightly,  so  as  to  tilt  the  needle  nearer  to 
or  farther  from  one  of  the  coils. 

The  Differential  Galvanoirieter. — This  is  a  form  of  galva- 
nometer in  which  a  magnetic  needle  is  suspended  between  two 
coils  of  equal  resistance  so  wound  as  to  tend  to  deflect  the  needle 
in  opposite  directions.     The  needle  of  a  differential  galvanometer 


GA  L  VA  NOME  TERS  45S 


shows  no  deflection  when  two  equal  currents  are  sent  through  the 
coils  in  opposite  directions,  since  under  these  conditions,  each 
coil  neutralizes  the  other's  effects.  Such  instruments  may  be 
used  in  comparing  resistances,  although  the  Wheatstone  bridge, 
in  most  cases,  affords  a  preferable  method. 

Ones.  What  is  the  special  use  of  the  differential  gal- 
vanometer ? 

Ans.     It  is  used  for  comparing  two  currents. 

Ques.    What  is  the  method  of  comparing  currents? 

Ans.  If  two  equal  currents  be  sent  in  opposite  directions 
through  the  coils  of  the  galvanometer,  the  needle  will  not  move ; 
if  the  currents  be  unequal,  the  needle  will  be  deflected  by  the 
stronger  of  them  with  an  intensity  corresponding  to  the  difference 
of  the  strengths  of  the  two  currents. 

Ques.     How  are  the  coils  adjusted? 

Ans.  This  is  done  by  coupling  them  in  series  in  such  a  way 
that  they  tend  to  turn  the  needle  in  opposite  directions,  and  when 
a  current  is  passing  through  them,  they  are  moved  nearer  to  the 
needle  or  farther  from  it  until  the  needle  stands  at  zero  with  an^- 
current. 

If  the  coils  be  not  movable,  a  turn  or  more  can  be  unwound  from  the 
coil  giving  the  greatest  magnetic  effect  until  a  balance  is  obtained,  the 
wire  so  unwound  can  then  be  coiled  in  the  base  of  the  instrument. 


Ballistic  Galvanometer.— This  type  of  galvanometer  is 
designed  to  measure  the  strength  of  momentary  currents,  such 
for  instance,  as  the  discharge  of  a  condenser.  In  construction 
the  magnetic  system  is  given  considerable  weight,  and  arranged 
to  give  the  least  possible  damping  efect. 


454 


IIA  IVKINS  ELECTRICITY 


The  term  "damping  effect"  means  the  offering  of  a  retarding  force 
to  control  swinging  vibrations,  such  as  the  movements  of  a  galvanometer 
needle,  and  to  bring  them  quickly  to  rest. 

If  a  momentary  current  be  passed  through  a  balHstic  galva- 
nometer, the  impulse  given  to  the  needle  does  not  cause  appre- 
ciable movement  to  the  magnetic  system  until  the  cturent  ceases, 
o'^'ing  to  the  inertia  of  the  heavy  moving  parts,  the  result  being 
a  slow  swing  of  the  needle. 


rRonnrr  unAMt 

S.  C.  State  CoUt^ 


Pig.  526. — Queen  dead  beat  and  ballistic  reflecting  galvanometer.  As  illustrated,  the  coils  are 
easily  removable  and  enclose  a  heavy  block  of  copper  fixed  in  a  central  fork.  In  a  cylin- 
drical hole  bored  in  this  block  hangs  the  bell  magnet  which  n-ith  its  mirror  is  suspended 
by  a  long  cocoon  fibre,  and  the  eddy  currents  induced  in  the  copper  bring  the  system  quickly 
to  rest  after  a  deflection.  By  lifting  the  copper  block  out  of  the  frame  the  instrume  *•  is 
made  ballistic.    The  instrument  is  made  with  coils  of  any  desired  resistance  up  to  1 ,000  ohms. 


Oues.     What  name  is  given  to  the  swing  of  a  ballistic 
galvanometer  needle?  ^ 

Ans.     It  is  called  the  kick.  ^^  Cr_.-rfl^ 

Oues.     How  is  the  current  measured  ? 

Ans.     As  the  needle  swings  slowly  around  it  adds  up,  as  it 


GALVANOMETERS 


455 


were,  the  varying  impulses  received  during  the  passage  of  the 
momentary  current,  and  the  quantity  of  electricity  that  has  passed 
is  proportional  to  the  sine  of  half  the  angle  of  the  first  swing  or  kick. 

If  a  reflecting  method  be  used  with  a  straight  scale,  the  observed 
deflection  depends  upon  the  tangent  of  twice  the  angle  of  movement 
of  the  needle.  For  small  deflections,  however,  the  change  of  flux  can  be 
taken  as  directly  proportional  to  the  observed  deflection. 


Fig.  527. — Thompson  galvanometer  with  mirror  reflecting  system  for  reading  the  deflections 
of  a  galvanometer  needle  by  the  movements  of  a  spot  of  light  reflected  from  a  mirror 
attached  to  the  needle  or  movable  magnetic  system. 


Use  of  Mirrors  in  Galvanometers. — In  order  that  small 
currents  may  be  measured  accurately,  some  means  must  be  pro- 
vided to  easily  read  a  small  deflection  of  the  needle.  Accordingly, 
it  is  desirable  that  the  pointer  be  very  long  so  that  a  large  number 
of  scale  divisions  may  correspond  to  small  deflections.  In 
construction,  since  sensitive  galvanometers  must  be  made  with 
the  moving  parts  of  little  weight,  it  would  not  do  to  use  a  long 
needle,  hence  a  ray  of  light  is  used  instead,  which  is  reflected  on  a 
distant  scale  by  a  small  mirror  attached  to  the  moving  part. 


456 


HA  WKIXS  ELECTRICITY 


In  the  Thompson  mirror  reflecting  galvanometer,  as  shown  in  fig.  528, 
a  small  vertical  slit  is  cut  in  the  lamp  screen  below  the  scale,  and  the  ray 
of  light  from  the  lamp,  passing  through  the  slit,  strikes  the  mirror 
which  is  about  three  feet  distant,  and  which  reflects  tne  beam  back  to 
the  scale.  It  should  be  noted  that  the  angle  between  the  original  ray 
of  light  and  the  reflected  ray  is  twice  the  angle  of  the  deflection  of  the 
mirror;  the  deflections  of  the  ray  of  light  on  the  scale,  however,  are 
practically  proportional  to  the  strength  of  currents  through  the  instru- 
ment. The  mirror  arrangement  as  shown  in  fig.  528,  requires  a  darkened 
room  for  its  operation,  but  such  is  not  necessary  when  a  telescope  is  used 
as  in  fig.  529.  Here  the  scale  readings  are  reflected  in  the  mirror  and  their 
value  observ'ed  by  the  telescope  wiQiout  artificial  light. 


TELESCOPE 


\ 


(fr-SCALE 


MIRROR 


^^^ MIRROR  ON 
GALVANOMETER  COIL 


Pig.  528. — Telescope  method  of  reading  galv.inometer  deflections  by  reflection  of  scale  reading 
in  mirror.  Here  two  mirrors  are  used,  but  in  most  cases  the  telescope  is  pointed  directh 
toward  the_  mirror  on  galvanometer  shown  in  fig.  527,  because  the  two  mirror  s>-stem,  as 
illustrated  in  the  figure,  is  used  on  ix>rtable  galvanometers  since  it  is  the  more  compact. 


Damping. — This  relates  to  the  checking  or  reduction  of  oscil- 
lations. Thus,  a  galvanometer  is  said  to  be  damped  when  so 
constructed  that  any  oscillations  of  the  pointer  which  may  be 
started,  rapidly  die  away.  Galvanometers  are  frequently  pro- 
vided with  damping  devices  for  the  purpose  of  annulling  thesq 


GALVANOMETERS 


457 


oscillations,  thus  causing  the  moving  part  to  assume  its  final 
position  as  quickly  as  possible. 

Sometimes  the  instrument  is  fitted  with  a  damping  coil,  or 
closed  coil  so  arranged  with  respect  to  the  moving  system  that 
the  oscillations  of  the  latter  give  rise  to  electric  currents  in  the 
closed  coil,  whereby  energy  is  dissipated.  Again,  air  vanes  are 
employed,  but  anything  in  the  nature  of  solid  friction  cannot  be 
used. 


Figs.  529  and  530. — Galvanometer  lamp  and  scale  for  individual  use.  The  scale  is  etched  on  a 
ground  glass  strip  6  centimeters  vnde  by  60  centimeters  long  with  long  centimeter  divisions 
and  short  millimeter  divisions  the  entire  length,  reading  both  waysfrom  zero  in  the  center. 
It  is  mounted  in  an  adjustable  wooden  frame.  A  straight  filament  lamp  (110  volts)  is 
enclosed  in  a  metal  hood  japanned  black  to  cut  out  all  reflected  light.  This  form  of  filament 
makes  a  single  brilliant  line  on  the  scale,  enabling  closer  readings  than  the  "spot  of  light" 
arrangement.    The  lamp  hood  is  adjustable  to  any  desired  height  on  the  support  rod. 


D'Arsonval  Galvanometer.— This  instrument  has  a  movable 
coil  in  place  of  a  needle,  and  its  operation  depends  upon  the 
principle  that  if  a  flat  coil  of  wire  be  suspended  with  its  axis 
perpendicular  to  a  strong  magnetic  field,  it  will  be  deflected  when- 
ever a  current  of  electricity  passes  through  it. 


458 


HA  WKINS  ELECTRICITY 


Oues.  Describe  the  construction  of  a  D'Arsonval  gal- 
vanometer. 

Ans.  The  essential  featttres  are  shown  in  figs.  532  and  533. 
The  coil,  which  is  rectangular  in  section  is  wound  upon  a  copper 
form,  and  suspended  between  a  permanent  magnet  by  fine  wires 
to  the  points  A  and  B.  The  magnet  has  its  poles  at  N  and  S.  It 
is  a  soft  iron  cylinder  fixed  between  the  poles  in  order  to  intensifj' 
the  magnetic  field  across  the  air  gaps  in  which  the  coil  moves. 


Fig.  Ml. — Queen  reading  "--:-:i-jpe.  This  arrar.genezt  is  utilized  tomeasurethe  deflections 
of  a  galvanometer  ha\-ing  suspended  mirror  moving  system.  It  consists  of  a  reading 
telescope  mounted  as  illustrated  with  a  millimeter  scale,  ha^•ing  a  length  of  50  centimeters. 
In  use.  the  image  of  the  scale  is  seen  in  the  galvanometer  mirror  through  the  telescoi>e.  The 
eye  piece  of  the  telescope  has  a  cross  hair  which  acts  as  a  reference  line  so  that  by  noting 
the  particular  di\'ision  on  the  scale  when  the  galvanometer  is  at  rest,  the  amount  of  deflec 
tion  can  be  readily  obser%-ed  when  the  galvanometer  is  deflected.  The  instrument  has  all 
the  necessary  adjustments  to  set  it  up  quickly  and  for  bringing  the  cross  hair  and  scale 
in  focus.     It  is  generally  placed  at  a  distance  of  one  meter  from  the  galvanometer  mirror. 

Oues.     Explain  its  operation. 

Ans.  An  enlarged  horizontal  cross  section  of  the  galvanome- 
ter on  line  XY  is  shown  in  fig.  533.  The  current  is  flowing  in 
the  coil  as  in  fig.  532,  up  on  the  left  side  and  down  on  the  right. 


GALVANOMETERS 


459 


The  position  of  the  coil  when  no  current  is  flowing  is  indicated 
by  n'  s'.  By  applying  the  law  of  mutual  action  between  mag- 
netic poles,  it  is  seen  that  when  the  current  is  applied,  the  poles 
developed  at  n'  s'  will  move  into  the  position  n"  s" .  See  fig.  119. 

Oues.     How  is  the  coil  affected  by  a  change  in  the  direc- 
tion of  the  current? 

Ans.     The  polarity  of  the  coil  is  reversed  and  consequently 
the  direction  of  the  deflection. 

-♦A 


Figs.  532  and  533. — Diagrams  showing  essential  features  of  construction  and  principle  of 
operation  of  D' Arson val  galvanometer. 

Oues.  Upon  what  does  the  sensitiveness  of  the  instru- 
ment depend  ? 

Ans.  Upon  the  strength  of  the  field  of  the  permanent  magnet, 
the  number  of  turns  in  the  suspended  coil,  and  the  torsion  of  the 
wires  by  which  it  is  suspended. 

Oues.    When  is  this  galvanometer  called  "  dead  beat  "  ? 

Ans.  When  the  construction  is  such  that  the  moving  part 
comes  quickly  to  rest  without  a  series  of  diminishing  vibrations. 


4G0 


HAWKINS  ELECTRICITY 


Fios.  534  to  536. — Queen  hori- 
zontal magnet  D'Arsonval 
galvanometer  with  telescope 
and  scale.  Itis  very  sensitive 
and  is  used  in  many  electrical 
measurements,  including  com- 
mercial testing,  such  as  meas- 
uring insulation  of  cables, 
fault  location,  etc.  It  is  not 
affected  by  surrounding  mag- 
netic disturbances,  and  may, 
therefore,  be  used  in  proximity 
to  dynamos  and  switchboards. 
The  instrument  has  a  pair  of 
binding  post  terminals,  one 
of  which  connects  to  a  bottom 
spiral  of  the  system  and  the 
other  forms  a  junction  with 
the  top  of  the  tube  holding  the 
system,  forming  a  complete 
circuit  through  the  coil.    The  '^ 

tube  containing  the  system  may  be  readily  removed  from  the  magnet  and  another  tube  hav- 
ing a  different  system  inserted  as  is  required  for  various  kinds  of  electrical  measurement. 
The  entire  system  with  its  suspension  may  be  inspected  by  the  removal  of  a  thumb  screw. 
To  inspect  interior  of  tube  first  be  sure  that  the  screw  B  is  turned  so  that  the  coil  is  clamped. 
Entirely  remove  screw  C,  and,  holding  the  outside  tube  near  the  window,  press  firmly  with 
the  finger  on  the  extreme  top  of  the  suspension  support.  The  inside  rib,  with  complete 
suspension,  will  draw  from  the  tube,  and  the  working  parts  can  be  fully  inspected.  Care- 
fully return  sarne  to  its  original  position  in  tube,  setting  tight  the  screw  C.  The  galva- 
nometer is  designed  so  that  the  coil  is  clamped  in  position  when  the  galvanometer  must  be 
transported.  The  insulation  of  the  galvanometer  terminals  and  binding  posts  is  such  as  to 
guard  against  any  possible  leakage.  As  a  further  protection,  each  levelling  screw  is  provided 
with  a  hard  rubber  insulator.  This  feature  is  essential  since,  in  making  insulation  measure- 
ments, the  operator  wishes  to  be  assured  that  the  deflection  being  obtained  is  the  result  of 
leakage  upon  the  cable  or  wire  being  measured  and  not  leakage  between  the  galvanometer 
terminals.  _  The  galvanometer  is  pro\'ided  with  an  attached  telescope  and  scale  for  noting 
the  deflections.  The  deflections  produced  by  this  galvanometer  are  proportional  to  the  cur- 
rent.    To  facilitate  quickly  setting  up  the  instrument,  two  way  levels  are  provided. 


GA  L  T  VI  NOME  TERS  40 1 


Oues.    What  causes  this? 

Ans.  The  instrument  is  made  dead  beat  by  winding  the  coil 
on  a  copper  or  aluminum  frame,  so  that  when  in  operation,  cur- 
rents are  induced  in  the  frame  by  the  motion  of  the  coil  in  the 
magnetic  field;  these  currents  oppose  the  motion  of  the  coil. 

Oues.  For  what  service  is  the  D'Arsonval  galvanometer 
adapted? 

Ans.  It  is  desirable  for  general  use  as  it  is  not  much  affected 
by  changes  in  the  magnetic  field.  It  may  be  made  with  high 
enough  period  and  sensibilit}'  to  be  satisfactory  as  a  ballistic 
instrument,  but  for  extreme  sensibility  an  instrument  of  the 
astatic  type  is  more  generally  used. 

Galvanometer  "  Constant  "  or  "  Figure  of  Merit." — In 

order  that  a  galvanometer  shall  be  of  value  as  a  measiuing  in- 
strument, the  relation  between  the  current  and  the  deflection 
produced  by  it  must  be  known.  This  may  be  obtained  experi- 
mentally by  determining  the  value  of  the  current  required  to 
produce  one  scale  division.  The  galvanometer  constant  then 
may  be  defined  as  the  resistance  through  which  the  galvanometer 
will  give  a  deflection  of  one  scale  division  when  the  current  applied 
is  at  a  pressure  of  one  volt. 

Accordingl}^  the  deflection  as  indicated  on  the  scale  must  be 
multiplied  by  its  constant  or  figure  of  merit,  in  order  to  obtain 
the  correct  reading.  If  the  scale  readings  be  not  directly  pro- 
portional to  the  quantity  to  be  measured,  the  law  of  the  instru- 
ment must  also  be  considered. 

Thus  in  a  tangent  galvanometer  as  previously  explained 

I  =  K  tan  ^ 

where  I  =  current,  <p  the  deflection  or  scale  reading,  and  K  the  gal- 
vanometer constant. 


462 


HAWKINS  ELECTRICITY 


Galvanometer  Shunts. — The  sensitiveness  of  a  galvanometer 
used  for  measuring  current  may  be  reduced  to  any  desired  extent 
by  connecting  a  resistance  of  knovm  value  in  parallel  \\nth  it. 
Thus,  if  it  be  desired  to  measure  a  current  greater  than  can  be 


GALVANOMETER 


r 


I   SHUNT 


-i   I 


Fig.  537. — EHagrajn  showing  method  of  connecting  galvanometer  shunt.    By  the  use  of  a  shunt 
the  range  of  measurement  of  a  galvanometer  can  be  greatly  increased. 


Fig.  538. — Diagram  of  a  form  of  universal  shunt  box  for  use  with  galvanometers  of  widely 
different  resistances.  The  galvanometer,  as  indicated  at  G.  is  connected  across  the  ends 
of  a  series  of  resistances  AB.  The  main  wires  are  connected,  one  to  end  A  of  the  series  and 
the  other  to  a  travelling  point  whose  position  is  varied  by  means  of  plugs  or  by  a  dial  switch. 


GALVANOMETERS  463 


measured  directly  by  the  galvanometer,  a  part  of  the  current 
can  be  sent  through  the  resistance  or  shunt,  and  the  total  value 
of  the  current  calculated. 

A  galvanometer  shunt  bears  a  definite  ratio  to  the  resistance 
of  the  galvanometer,  being  usually  adjusted  so  that  only  .1, 
.01,  or  .001  part  of  the  current  passes  through  the  galvanometer. 

The  degree  in  which  a  shunt  increases  the  range  of  deflection 
of  a  galvanometer  is  called  its  "multiplying  power." 


Pig.  539. — Ayrton-Mather  universal  shunt.  This  shunt  may  be  used  with  any  galvanometer. 
The  total  resistance  is  10,000  ohms,  with  shunt  powers  of  1,  5,  10,  50,  100,  500,  and  l.OOo! 
It  is  also  fitted  with  positions  in  which  the  galvanometer  is  shorted  and  off.'  The  coils 
are  of  constantan  wire. 

If  .1  of  the  current  flowing,  passed  through  the  galvanometer  and  .9 
through  the  shunt,  then  the  current  in  the  circuit  would  be  ten  times 
that  through  the  galvanometer.  Accordingly  the  current  in  the  gal- 
vanometer must  be  multiplied  by  the  multiplying  power  of  the  shunt  to 
obtain  the  true  value  of  the  current  in  the  circuit. 

In  order  to  determine  the  resistance  necessary  to  be  used 
with  a  certain  galvanometer,  the  resistance  of  the  latter  is  to  be 
divided  by  the  multiplying  power  desired  less  one. 


464  HAWKIXS  ELECTRICITY 

EXAMPLE. — What  must  be  the  resistance  of  a  shunt  for  a  galva- 
nometer of  2.000  ohms  resistance  where  only  one  fifth  of  the  current  is 
to  pass  through  the  gal\"anometer? 

The  multipljTng  power  less  one  is 

5-1  =  4 
and  the  required  resistance  is 

2,000  -T-  4  =  500  ohms. 

When  it  is  essential  that  the  total  resistance  of  the  circuit 
should  not  be  altered  by  an  alternation  of  the  galvanometer 
shunt,  a  compensating  box  should  be  used  which  automatically 
inserts  a  resistance  for  each  shtmt  in  series  with  the  shunted 
galvanometer  to  bring  the  total  resistance  up  equal  to  the 
unshunted  value.  Thus  the  current  in  the  main  circuit  is  not 
altered. 


TESTING  AND    TESTING  APPARATUS  465 


CHAPTER  XXVII 
TESTING  AND  TESTING   APPARATUS 


The  practical  electrician  frequently  has  to  make  tests  of 
various  kinds  which  require  the  rapid  and  accurate  measurement 
of  voltage,  current  and  resistance.  It  is  therefore  essential  that 
he  understand  the  methods  employed  in  testing  and  the  operation 
of  the  instruments  used. 

Most  tests  are  made  with  a  galvanometer,  and  the  devices, 
such  as  resistances,  switches,  etc.,  which  are  used  in  connection 
with  the  galvanometer  may  be  obtained  put  up  in  a  neat  and 
substantial  box  together  with  the  galvanometer,  the  combination 
being  called  a  "  testing  set."  Numerous  forms  of  testing  set  are 
illustrated  in  this  chapter. 

The  construction,  use,  and  operation  of  the  various  types  of 
galvanometer  have  been  explained  in  chapter  twenty-six. 
Ammeters  and  voltmeters,  which  are  simply  special  forms  of 
galvanometer,  and  which  are  largely  used  are  fully  described 
in  the  preceding  chapter. 

Pressure  Measurement. — An  electromotive  force  has  been 
defined  as  that  which  causes  or  tends  to  cause  a  current;  it  is 
analogous  to  water  pressure ;  potential  difference  corresponds  to 
difference  of  level.     The  total  electromotive  force  of  a  circuit  is 


A66 


HAWKINS  ELECTRICITY 


independent  of  resistance  or  current,  and  cannot  be  limited  to 
mean  the  fall  of  pressure  between  any  two  points,  as  for  in- 
stance the  terminals  of  a  battery. 

If  the  pressure  of  a  battery  be  two  volts  when  measured  on  open 
circuit  by  a  static  voltmeter,  there  will  still  be  two  volts  on  closed 
circuit,  but  there  will  now  be  a  loss  of  pressure  through  the  internal 
resistance  of  the  battery  and  the  voltage  across  the  terminals  will  be  less 
than  the  total  voltage.  The  static  voltmeter,  never  closing  the  circuit, 
actually  measures  the  total  voltage. 


Zinc  sulphate 

CRVSTMS 

MEffCUROUS  AND  — 
SULPHATE  AND  ZINC 
SULPHATE  PASTE 


MEI?CUI?Y' 


ZINC  SULPHATE 

SOLUTION 


ZINC   SULPHATE 
CRYSTALS 


AMALOAM  OF  ZINC 
AND  MERCYRY 


/m^ 


Fig.  540. — Clark  cell  (Kahle's  modification  of  the  Rayleigh  H  form),  the  standard  for  the 
International  volt.  The  cell  has  for  its  positive  electrode,  mercur>-,  and  for  its  negative 
electrode,  amalgamated  zinc.  The  electrolyte  consists  of  a  saturated  solution  of  zinc 
sulphate  and  mercurous  sulphate.  The  pressure  is  1.434  volts  at  15°C..  and  between  10°C. 
and  25°C.  the  pressure  decreases  .001 15  of  a  volt  for  each  increase  of  1°C.  The  containing 
glass  vessel  consists  of  two  limbs,  closed  at  bottom  and  joined  above  to  a  common  neck 
fitted  with  a  ground  glass  stopper.  The  diameter  of  the  hmbs  should  be  at  least  2  cms., 
and  their  length  at  least  3  cms.  The  neck  should  be  not  less  than  1.5  cms.  in  diameter.  At 
the  bottom  of  each  limb  a  platinum  wire  of  about  .4  mm.  in  diameter  is  sealed  through  the 
glass.  To  set  up  the  cell,  place  mercury  in  one  limb,  and  in  the  other  hot  liquid  amalgam, 
containing  90  parts  mercury  and  10  parts  zinc.  The  platinum  wires  at  the  bottom  must 
be  completely  covered  by  the  mercury  and  the  amalgam,  respectively.  On  the  mercury, 
place  a  layer  1  cm.  thick  of  the  zinc  and  mercurous  sulphate  paste.  Both  this  paste  and 
the  zinc  amalgam  must  be  covered  with  a  layer  of  the  neutral  zinc  sulphate  crystals  1  cm. 
thick.  The  whole  vessel  must  then  be  filled  with  the  saturated  zinc  sulphate  solution, 
and  the  stopper  inserted  so  that  it  shall  just  touch  it,  lea\'ing,  however,  a  small  bubble 
to  guard  against  breakage  when  the  temi)erature  rises.  Before  finally  inserting  the  glass 
stopper  a  strong  alcoholic  solution  of  shellac  is  applied  to  the  upper  edge,  after  which 
the  stopper  is  pressed  firmly  in  place. 


TESTING  AND    TESTING  APPARATUS 


467 


Oues.     What    error    is    introduced    in    measuring    the 
pressure  of  a  battery  with  an  ordinary  voltmeter? 

Ans.     Since  the  measurement  is  made  on  closed  circuit  the 
reading  does  not  give  the  total  pressure  of  the  battery. 

The  error  is  very  slight  because  the  resistance  of  the  voltmeter  is  very 
high  and  the  current  so  small  that  the  loss  of  pressure  in  the  battery 
can  be  neglected. 


Fig.  541. — Weston  Cadmium  Cell.  It  is  made  in  two  forms;  one  known  as  the  Weston  normal 
cell,  in  which  the  solution  of  cadmium  sulphate  is  saturated  at  all  temperatures  at  which 
the  cell  may  be  used.  The  other,  known  as  the  Weston  standard  cell,  in  which  the  cadmium 
sulphate  solution  is  unsaturated  at  all  temperatures  above  4°  C.  The  Weston  normal 
cell,  or  saturated  form  is  slightly  affected  by  changes  in  temperature,  but,  on  account  of 
the  fact  that  it  can  be  accurately  reproduced,  it  was  adopted  by  the  London  Conference 
in  1908,  as  a  convenient  voltage  standard.  The  value  of  its  voltage  suggested  by  the 
committee  of  the  London  Conference  on  Electrical  Units  and  Standards,  and  adopted 
by  the  Bureau  of  Standards  at  Washington,  Jan.  1st,  1911,  is  1.0183  International  volts 
at  20°  C.     At  any  other  temperature  its  voltage  is: 

Ej  =  Ejo-  .0000406  (t-  20)  -.00000095  (t  -  20)'  +  .0000000  (t-  20"')» 
The  Weston  standard  cell,  or  unsaturated  form  is  practically  unaffected  by  changes  in 
temperature  and  is  the  form  most  commonly  used  for  laboratory  work  and  general  testing. 
The  average  pressure  of  this  form  i$  1.01S7  Int.  volts. 


468 


HAWKINS  ELECTRICITY 


Ques.     Define  the  International  volt. 

Ans.  It  is  the  electromotive  jorce  that,  steadily  applied  to  a  con- 
ductor whose  resistance  is  one  International  ohm,  will  produce  a 
current  of  one  I  titer  national  ampere,  and  which  is  respresented 


^5?^^^55^^J^%5!^^;:^^^^^55??^ 


Pigs.  542  and  543. — Diagrams  showing  hydraulic  aaa]og>'  illustrating  the  difference  between 
amperes  and  coulombs.  If  the  current  strength  in  fig.  543  be  one  ampere,  the  quantity 
of  electricity  passing  any  point  in  the  circmt  per  hour  is  1  X  60  X  60  =  3.600  coulombs. 
The  rale  of  current  flow  of  one  ampere  in  fig.  543  may  be  compared  to  the  rate  of  discharge 
of  a  pump  as  in  fig.  512.  Assuming  the  pump  to  be  of  such  sire  that  it  discharges  a  gallon 
per  stroke  and  is  making  60  strokes  per  minute,  the  quantity  of  water  dis.-harged  per  hour 
(coulombs  in  fig.  543)  is  1  X  60  X  60  =  3.600  gallons.  Following  the  analogy-  further  (Ln 
fig.  543).  the  pressure  of  one  volt  is  required  to  force  the  electricity  through  the  resistance 
of  one  ohm  between  the  terminals  A  and  B.  In  fig.  542.  the  boiler  must  furnish  steam 
pressure  on  the  pump  piston  to  overcome  the  friction  (resistance)  offered  by  the  pip* 
and  raise  the  water  from  the  lower  level  A'  to  the  higher  level  B'.  The  difference  of 
pres'sure  between  A  and  B  in  the  electric  circuit  corresponds  to  the  difference  of  pressure 
between  A'  and  B'.  The  cell  furnishes  the  energ>-  to  move  the  current  by  maintaining  a 
difference  of  pressure  at  its  terminals  C  and  D;  similarly,  the  boiler  furnishes  energ>- 
to  raise  the  water  by  maintaining  a  difference  of  pressure  between  the  steam  pipe  C  and 
exhaust  pipe  D'. 

sufficiently  well  for  practical  use  by  of  the  voltage  between 

l,4o-i 

the  poles  of  the  Clark  cell  at  a  temperature  of  15°  C,  when  prepared 

as  in  fig.  540. 


The  relation  between  the  units  volt,  ampere  and  ohm,  are  shown 
graphically  in  figs.  542  and  543. 


TESTING  AND   TESTING  APPARATUS 


469 


Current  Measurement. — It  is  necessary  to  adopt  some 
arbitrary  standard  in  order  to  compare  currents  of  different 
strengths.  The  term  strength  of  a  current,  or  current  strength 
means  the  rate  of  flow  past  any  point  in  the  circuit  in  a  given 
unit  of  time.  The  unit  of  current,  called  the  ampere,  is  defined 
as  the  unvarying  current  which,  when  passed  through  a  solution  of 
nitrate  of  silver  in  ivater  (15  per  cent,  by  weight  of  the  nitrate) 
deposits  silver  at  the  rate  ^/  .001118  gramme  per  second. 


Fig.  544. — Queen  weight  voltameter  for  determining  the  strength  of  current  by  the  weight  of 
metal  deposited  in  a  given  time.  The  two  outside  plates  form  the  anode  and  are  joined 
together  and  to  one  binding  post,  while  the  cathode  is  placed  between  them  and  connected 
to  the  other  binding  post.  The  cathode  thus  recives  a  deposit  on  both  sides.  An  adjustable 
arm  serves  to  lower  the  plates  into  the  electrolyte.  To  calculate  the  strength  of  an  un- 
known current  which  has  passed  through  a  weight  voltameter,  divide  the  gain  in  weight  by 
the  number  of  seconds  the  current  flows  through  the  instrument  and  by  the  weight  deposited 
by  one  ampere  in  one  second.  That  is,  current  strength  in  amperes  =  gain  in  weight  -j- 
(time  in  seconds  X  .0003286). 


Oues.     How  much  copper  or  zinc  will  one  ampere  de- 
posit in  one  second  ? 

Ans.     .0003286  gramme  of  copper  in  a  copper  voltameter,  or 
.0003386  gramme  of  zinc  in  a  zinc  voltameter. 


470 


HAWKINS  ELECTRICITY 


Oues.  What  is  the  difference  between  an  ampere  and 
a  coulomb? 

Ans.  An  ampere  is  the  unit  rate  oj flow  of  the  current,  and  a 
coulomb  is  the  unit  quantity  of  electricity,  that  is,  the  ampere  is 
the  rate  of  current  flow  that  will  deposit  .0003286  grammes  of 
copper  in  one  second  and  a  coulomb  is  the  quantity  of  electricity 
that  passes  a  given  point  in  one  second  when  the  current  strength 
is  one  ampere.      In  other  words  a  coulomb  is  one  ampere  second. 


Fig.  .545. — Gas  voltameter  for  determining  the  strength  of 
current  by  the  volume  of  gas  evolved.  To  use,  connect 
up  as  shown  in  the  illustration.  Adjust  so  that  the  zero 
position  of  the  burette  is  about  one-half  inch  below  the 
level  of  the  top  of  the  U  tube.  Pour  acidulated  water 
into  the  mouth  of  the  burette  till  the  water  in  the  U 
tube  is  about  one-half  inch  from  the  top.  With  the. 
electrodes  inserted  through  the  corks,  carefully  place 
each  one  in  position  .by  giving  a  slight  twist  to  the  right 
as  the  cork  enters.  The  water  level  in  the  U  tube  and 
burette  should  now  be  the  same,  or  further  adjustment 
must  be  made  to  attain  this  result.  The  level  in  the 
burette  does  not  necessarily  have  to  correspond  with  the 
zero  graduation,  but  must  not  be  below  it.  Unclamp 
the  burette  and  hold  it  nearly  horizontal.  The  liquid 
will  not  run  out  if  the  corks  be  tight,  so  that  this  is  the 
air  leakage  test.  Attach  the  connectors  and  wires  from 
the  current  source  (which  should  have  a  pressure  of 
2  or  more  volts)  placing  a  switch  in  the  circuit.  When 
the  switch  is  closed,  bubbles  of  gas  will  rise  in  the 
U  tube  from  both  electrodes,  displacing  the  water  and 
forcing  it  up  the  burette.  Hydrogen  will  be  liberated 
over  the  negative  electrode,  and  oxygen  over  the  pos- 
itive electrode  in  the  proportion  of  twice  as  much  hydro- 
gen as  oxygen.  To  calculate  the  current  strength,  divide 
the  volume  of  gas  liberated  by  the  time  in  seconds,  and  by 
the  volume  of  gas  liberated  (in  cubic  centimeters)  by  one 
ampere  in  one  second  and  by  .1733;  that  is:  amperes  = 
volume  of  gas  liberated  -H  (time  in  seconds  X  .  1733). 


EXAMPLE. — If  an  arc  lamp  require  a  current  of  8  amperes,  how 
much  electricity  does  it  consume  per  hour? 

Since  one  coulomb  =  one  ampere  second,  the  quantity  of  electricity 
consumed  per  hour  is 

amperes  I    ^    {^^J^^ndsj    =  28,800  coulombs. 


TESTING  AND    TESTING  APPARATUS  471 

Voltameter. — A  voltameter  is  an  electrolytic  cell  employed  to 
measure  an  electric  current  by  the  amount  of  chemical  decom- 
position the  current  causes  in  passing  through  the  cell.  There 
are  two  classes  of  voltameter: 

1.  Weight  voltameters; 

2.  Gas  voltameters. 

Oues.  What  is  the  difference  between  these  two  classes 
of  voltameter? 

Ans.  In  one,  the  current  strength  is  determined  by  the 
weight  of  metal  deposited  or  weight  of  water  decomposed,  and 
in  the  other  by  the  volume  of  gas  liberated. 

Fig.  544  shows  a  weight  voltameter  and  fig.  545  a  gas  voltameter. 

Oues.  How  should  the  plates  of  a  weight  voltameter  be 
treated  before  vise? 

Ans.  They  must  be  thoroughly  cleaned  and  polished  with 
sandpaper,  the  sand  being  afterwards  removed  by  placing  them 
in  running  water.  The  fingers  must  not  be  placed  on  any  part  of  the 
plate  which  is  to  receive  the  deposit. 

Oues.  What  form  of  voltameter  has  been  selected  to 
measure  the  International  ampere? 

Ans.     The  silver  voltameter  arranged  as  here  specified: 

The  cathode  on  which  the  silver  is  to  be  deposited  shall  take  the 
form  of  a  platinum  bowl,  not  less  than  10  cms.  in  diameter,  and  from 
4  to  5  cms.  in  depth. 

The  anode  shall  be  a  disc  or  plate  of  pure  silver  some  30  sq.  cms.  in 
area,  and  2  or  3  cms.  in  thickness.  This  shall  be  supported  horizon- 
tally in  the  liquid  near  the  top  of  the  solution  by  a  silver  rod  riveted 
through  its  center. 

To  prevent  the  disintegrated  silver  which  is  formed  on  the  anode 
falling  upon  the  cathode,  the  anode  shall  be  wrapped  around  with  pure 
filter  paper,  secured  at  the  back  by  suitable  folding. 

The  liquid  shall  consist  of  a  neutral  solution  of  pure  silver  nitrate 
containing  about  15  parts  by  weight  of  the  nitrate  to  85  parts  of  water. 


472 


HAWKINS  ELECTRICITY 


Oues.  What  is  the  value  of  the  International  ampere 
as  lAeasured  with  the  silver  voltameter? 

Ans.  The  International  ampere  is  represented  sufficiently 
well  for  practical  use  by  the  unvarying  current  which,  when 
passed  through  a  silver  voltameter  (as  described  above)  deposits 
silver  at  the  rate  of  .001118  gramme  per  second. 


Pig.  546. — Single  contact  and  short  circuitinc  key.  This  key  is  intended  e.<^peciaUy  for  use  with 
D'Arsonval  galvanometers  in  zero  deflection  methods.  The  key  is  connected  in  circuit 
vnXh.  the  galvanometer  so  that  whenever  the  key  is  not  depressed,  the  galvanometer  is 
short  circuited,  and  its  oscillations  quickly  damped  out  by  the  currents  induced  in  its  coil. 
The  back  end  of  the  spring  is  held  in  a  slot  in  a  rubber  block  attached  to  the  base. 


Ohm's  Law  and  the  Ohm. — The  various  tests  here  de- 
scribed depend  for  their  truth  upon  the  definite  relation  existing 
between  the  electric  current,  its  pressure,  and  the  resistance 
which  the  circuit  offers  to  its  flow.  This  relation  was  fully  in- 
vestigated by  Ohm  in  1827.  Using  the  same  conductor,  he 
proved  not  only  that  the  current  varies  with  the  pressure,  but 
that  it  varies  in  direct  proportion. 

Ohm's  law  has  already  been  discussed  in  a  pre\dous  chapter 

and  the  several  ways  of  expressing  it  are  repeated  here  for  con . 

venience : 

volts 

1.  Amperes  = ; 

ohms 


TESTING  AND    TESTING  APPARATUS 


473 


2.  Volts 

3.  Ohms 


amperes  X  ohms; 

volts 
amperes 


Various  values  have  been  assigned,  from  time  to  time,  to 
the  ohm  or  unit  of  resistance,  the  unit  in  use  at  the  present  time 
being  known  as  the  International  ohm.     This  was  recommended 


Fig.  547. — Double  contact  key.  It  is  of  especial  value  in  connection  with  a  Wheatstone  bridge. 
When  used  with  the  latter  it  forms  a  combination  battery  and  galvanometer  key.  The 
battery  is  wired  to  the  top  leaves  of  the  key  and  the  galvanometer  to  the  lower  leaves. 
Hence,  when  operated,  the  battery  circuit  will  be  closed  before  the  galvanometer  circuit, 
as  it  is  desirable  to  avoid  undue  disturbance  of  the  needle. 


at  the  meeting  of  the  British  Association  in  1892,  was  adopted 
by  the  International  Electrical  Congress  held  in  Chicago  in  1893, 
and  was  legalized  for  use  in  the  United  States  by  act  of  Congress 
in  1894.  The  International  ohm  in  graphically  defined  in  fig.  548. 
The  previous  values  given  to  the  ohm  which  were  more  or  less 
generally  accepted  are  as  f oUows : 


471 


HAWKINS  ELECTRICITY 


The  Siemens'  Ohm. — A  resistance  due  to  a  column  of  mercury  100 
cm.  long  and  1  sq.  mm.  in  cross  section  at  0°  C. 

B.  A.  (British  Association)  Ohm. — A  resistance  due  to  a  column 
of  mercury  approximately  104.9  cm.  long  and  1  sq.  mm.  in  cross  section 
at  0"  C. 


-ISQ.MM 


Fig.  548. — The  international  ohm.  It  is  defined  as  the  resistance  of  14.452  grammes  of  mercury 
in  the  form  of  a  column  of  uniform  cross  section  106.3  centimeters  in  length,  at  a  temper- 
ature of  0°C.  This  is  approximately  equivalent  to  a  column  106.3  cm.  long,  having  a 
uniform  cross  section  of  1  sq.  mm.  In  the  figure  the  international  ohm  is  defined 
graphically.  The  resistance  of  the  external  circuit  and  the  standard  one  volt  cell  is 
assumed  to  be  zero. 


Legal  Ohm. — A  resistance  due  to  a  column  of  meicury  106  cm.  long 
and  1  sq.  mm.  in  cross  section  at  0°  C.  This  unit  was  adopted  by  the 
Paris  conference  of  1884. 


TESTING  AND    TESTING  APPARATUS 


475 


OHM  TABLE 

* 

Date 

Inter- 
national 
Ohm 

Legal 
Ohm 

B.  A. 

Ohm 

Siemens' 
Ohm 

International  Ohm. 

Legal  ()!im         

1893-4 
1884 
1864 

1. 
.9972 
.9866 
.9407 

1.0028 

1. 
.9894 
.9i34 

1.0136 
1.0107 
1. 
.9535 

1.0630 
1.0600 
1.0488 
1. 

B.  A.  Ohm 

Siemens'  Ohm 

Fig.  549. — Leeds  and  Northrup  one  ohm  standard  resistance  (Reichsanstalt  form);  ad- 
justed at  20°  C.  Low  resistance  standards  may  be  properly  divided  into  two  classes: 
1.  those  which  are  designed  primarily  as  resistance  standards,  and  2,  those  designed  as 
current  carrying  standards.  Those  of  the  first  mentioned  class  are  often  used  to  measure 
currents  up  to  their  capacity.  The  above  standa.d  has  both  pressure  and  current  termi- 
nals. The  binding  posts  for  the  former  are  mounted  on  high  posts  so  as  to  be  easily 
accessible  when  the  standard  is  immersed  in  oil.  When  used  as  a  resistance  standard  of 
precision,  it  should  not  be  subjected  to  a  current  of  more  than  one  ampere,  and  when 
used  as  a  current  carrying  standard  of  lesser  accuracy,  a  current  of  2  or  3  amperes  may 
be  used. 


•  NOTE. — In  the  above  table  to  reduce,  for  instance,  British  Association  ohms  to  Inter- 
national ohms,  multiply  by  .9866,  or  divide  by  1.0136;  to  reduce  legal  ohms  to  International 
ohms,  multiply  by  .9972.  or  divide  by  1.0028.  etc. 


470 


HA  WKINS  ELECTRICITY 


Practical  Standards  of  Resistance. — The  column  of 
mercury  as  shown  in  fig.  548,  is  the  recognized  standard  for 
resistance,  however,  in  practice,  it  is  not  convenient  to  compare 
resistances  with  such  a  piece  of  apparatus,  and  therefore  secondary 
standards  are  made  up  and  standardized  with  a  great  degree  of 
precision.  These  secondary  standards  are  made  of  wire.  The 
material  generally  used  being  manganin  or  platinoid. 


6MVAN0METEI? 

TWO  WAV  KFY 

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KNOWN   RESlST^NCE 

^               / 

1 

• 

UNKNOWN  RESISTANCE 

Fig.  550. — Direct  deflection  method  of  testing  resistances;  a  useful  and  siinple  method  which 
may  be  used  in  numerous  tests.  Galvanometer  readings  are  taken  through  the  knowTi.  and 
tinknown  resistances,  and  the  current  being  proportiozial  to  the  deflections,  the  \'alue  of  the 
unknown  resistance  is  easily  calculated. 


Resistance  Measurement. — Resistance  is  that  ii'hich  offers 
opposition  to  the  flow  of  electricity.  Ohm's  law  shows  that  the 
strength  of  the  current  falls  off  in  proportion  as  the  resistance  in 
the  circuit  increases.    This  gives  a  basis  for  measuring  resistance. 

There  are  various  methods  by  which  an  unknown  resistance 
may  be  measured,  as  by  the: 


1.  Direct  deflection  method; 

2.  Method  of  substitution; 

3.  Fall  of  potential  method; 

4.  Differential  galvanometer  method; 


TESTING  AND   TESTING  APPARATUS 


477 


5.  Drop  method; 

6.  Voltmeter  method ; 

7.  Wheatstone  bridge  method. 


Direct  Deflection  Method. — This  method  is  based  on  the 
fact  that  the  greater  the  current  through  a  galvanometer  the 
greater  the  deflection  of  the  needle ;  it  is  a  simple  method  and  is 
capable  of  extended  application. 

The  apparatus  required  consists  of  battery,  galvanometer, 
known  resistance,  and  double  contact  key.    The  connections  are 


Fig.  551. — Charge  and  discharge  key,  adapted 
to  condenser  testing  where  the  condenser  is 
to  be  alternately  charged  and  discharged. 
The  insulated  handle  enables  the  key  to  be 
used  without  insulating  the  body. 


Fig.  552. — Pohl  commutator.  This  is  equiv- 
alent  to  a  two  pole  double  throw  switch. 
The  depressions  in  the  base  are  filled  with 
mercury  into  which  the  contacts  dip  in 
closing  the  circuit. 


made  as  in  fig.  550.  The  known  resistance  is  put  in  circuit  with 
the  gah^anometer  and  after  noting  the  deflection,  the  key  is 
moved  so  as  to  cut  out  the  known  resistance  and  throw  into 
circuit  the  unknown  resistance.  The  deflection  of  the  galva- 
nometer is  again  noted  and  compared  with  the  first  deflection. 

If  the  deflections  be  proportional  to  the  current,  the  unknown 
resistance  will  be  as  many  times  the  known  resistance  as  the 
deflection  with  the  known  resistance  is  greater  than  the  deflection 
with  the  unknown  resistance. 


478 


HAWKINS  ELECTRICITY 


Method  of  Substitution. — This  is  the  simplest  method  of 
measuring  resistance.  The  resistance  to  be  measured  is  inserted 
in  series  with  a  galvanometer  and  some  constant  source  of 
current,  and  the  galvanometer  deflection  noted.  A  known  ad- 
justable resistance  is  then  substituted  for  the  unknown  and 
adjusted  till  the  same  deflection  is  again  obtained.  The  value 
of  the  adjustable  resistance  thus  obtained  is  equal  to  that  of 
the  resistance  being  tested. 


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6ALVAN0METER 


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BATTERY 

Hill 


Fig.  553. — Substitution  method  of  testing  resistances.  The  connections  and  apparatus  are  the 
same  as  in  fig.  550,  except  that  a  resistance  box  is  used  in  place  of  the  known  resistance. 
In  maJdng  the  test,  first  note  deflection  with  unknown  resistance  in  circuit,  then  press 
key  so  that  the  current  will  pass  through  the  resistance  box,  and  adjust  the  resistance  in 
the  box  so  that  the  deflection  of  the  galvanometer  is  about  the  same  as  with  the  unknown. 
Now  switch  from  one  circuit  to  the  other,  changing  the  resistance  in  the  box  until  equal 
deflections  are  obtained.  When  this  obtains,  the  resistance  in  the  box  is  the  same  as 
the  resistance  being  tested. 


Oues.    What  kind  of  adjustable  resistance  is  used  in 
making  the  above  test? 
Ans.     A  resistance  box. 


TESTING  AND    TESTING  APPARATUS 


479 


Oues.     Describe  a  resistance  box. 

Ans.  It  consists  of  a  box  containing  numerous  resistance 
coils  with  their  ends  connected  to  terminals  and  provided  with 
plugs  so  that  they  may  be  thrown  into  or  out  of  circuit  at  will, 
thus  varying  the  resistance  in  the  circuit. 


Figs.  554. — Ordinary  resistance  box.  It  contains  sets  of  standard  resistances  consisting  of 
coils  ot  insulated  wire  having  low  conductivity  and  small  temperature  coefficient.  The 
ends  of  the  coils  are  joined  to  the  section  of  the  bar  between  the  plugs.  The  insertion  of 
a  plug  cuts  out  a  coil.  In  using,  care  should  be  taken  to  put  the  plugs  in  with  a  slight 
twist  so  that  there  shall  be  no  resistance  introduced  by  poor  contact. 


Fall  of  Potential  Method. — This  is  a  very  simple  method 
of  measuring  resistances,  and  one  that  is  convenient  for  practical 
work  in  electrical  stations  because  it  requires  only  an  ammeter, 
voltmeter,  battery  and  switch — apparatus  to  be  found  in  every 
station.    The  connections  are  made  as  shown  in  fig.  555. 

In  making  the  test  the  ammeter  and  voltmeter  readings  are 
taken  at  the  same  time,  and  the  unknown  resistance  calculated 
from  Ohm's  law.     Accordingly,  since: 


480 


HAWKINS  ELECTRICITY 


amperes  = 


volts 


ohms 
solving  for  the  resistance, 

volts 


ohms  = 


amperes 


(1) 


(2) 


AMMETER 


VOLTMETER 


SWITCH 


Fig.  555. — Fall  of  potential  method  of  testing  resistances;  a  convenient  method  for  testing  at 
stations,  requiring  only  the  usual  instruments  to  be  found  at  a  station.  The  resistance 
of  the  voltmeter  must  be  verj-  high,  otherwise  the  test  must  be  made  as  in  fig.  556. 


EXAMPLE. — If  in  fig.  555  the  readings  show  6  volts  and  2  amperes 
how  many  ohms  is  the  resistance  being  tested? 


Substituting  in  formula  (2) 


ohms  =  - 


TESTING  AND   TESTING  APPARATUS 


481 


Ques.    Can  this  test  be  made  with  any  kind  of  voltmeter  ? 

Ans.  Its  resistance  must  be  very  high  to  avoid  error.  When 
a  voltmeter  having  small  resistance  is  used,  it  should  be  con- 
nected so  as  to  measure  the  fall  of  pressure  across  both  ammeter 
and  unknown  resistance  as  shown  in  fig.  556 


SWITCH 


BATTERY 


UNKNOWN  RESISTANCE 
VOLTMETER 


Fio.  556, — Fall  of  potential  method  of  testing  resistances;  diagram  showing  connections  for 
testing  with  low  resistance  voltmeter.  The  resistance  measured  with  this  connection  will  be 
the  sum  of  the  resistances  of  the  coil  and  the  ammeter.  The  resistance  of  the  ammeter 
is  usually  known  and  can  be  subtracted  from  the  sum  to  obtain  the  required  resistance. 


Differential  Galvanometer  Method. — This  is  what  is 
known  as  a  nil  or  zero  method,  that  is,  a  method  of  making 
electrical  measurements  in  which  comparison  is  made  between 
two  quantities  by  reducing  one  to  equality  with  the  other,  the 
absence  of  deflection  from  zero  of  the  instrument  scale  showing 
that  the  equality  has  been  obtained. 


482 


HAWKINS  ELECTRICITY 


The  test  is  made  with  a  differential  galvanometer,  and  re- 
sistance box  connected  as  in  fig.  557.  The  current  then  will 
di\'ide  so  that  part  of  it  flows  through  the  resistance  being  tested 
and  around  one  set  of  coils  of  the  galvanometer  while  the  other 
part  will  flow  through  the  resistance  box  and  the  other  set  of 
coils  as  indicated.  When  the  resistance  box  has  been  so  adjusted 
that  its  resistance  is  the  same  as  the  unknown  resistance  the 
current  in  the  two  branches  will  be  equal,  and  the  needle  of  the 
galvanometer  will  show  no  deflection. 


DIFFERENTIAL 

GALVANOMETER 


RESISTANCE  BOX 


^'6660 


I — K)      6      6      0      0      0 


> 


\ 


L^^ 


UNKNOWN  RESISTANCE 
•^WVWVVVW 


BATTERY 


Fig.  557. — Differential  galvanometer  method  of  testing  resistances.  In  making  the  test,  the 
resistance  bo.x  is  adjusted  till  the  galvanometer  needle  shows  no  deflection.  When  this 
condition  obtains,  the  resistance  in  circuit  in  the  resistance  box  is  equal  to  the  unknown 
resistance,  hence,  a  reading  of  the  bo.^c  gives  the  value  of  the  unknown  resistance. 


Oues.     What  name  is  given  to  this  method  of  testing? 

Ans.     It  is  called  a  zero  method,  distinguishing  it  from  de- 
flection methods. 

Oues.     For   what    kind    of   resistance    is    the   method 
adapted? 

Ans.     Since  it  is  a  nil  or  zero  method,  it  is  better  adapted  to 
the  measurement  of  non-inductive  than  of  inductive  resistances. 


TESTING  AND   TESTING  APPARATUS 


483 


Ques.     What  precaution  should  be  taken  with  inductive 
resistances? 

Ans.     The  current  must  be  allowed  to  flow  until  it  becomes 
steady  to  overcome  the  influence  of  self-induction. 

Oues.     What  may  be  said  with  respect  to  the  differential 
galvanometer  method? 

Ans.     With  an  accurate  instrument  it  is  very  reliable. 


TWO  WAY 
SWITCH 


KNOWN  RESISTANCE 


UNKNOWN  RESISTANCE 
■*>A\A/VvVvVvW — — " 


BATTERY 


I  I  I 


Fig.  558. — Drop  method  of  testing  resistances.  The  apparatus  is  connected  as  shown  and 
readings  taken  with  voltmeter  across  known  and  unknown  resistance.  The  unknown 
resistance  is  then  easily  calculated. 


Drop  Method. — This  is  a  convenient  method,  and  one  which 

may  be  used  for  measuring  either  high  or  low  resistances  with 
precision.  It  is  used  for  many  practical  measurements,  and 
requires  only  a  voltmeter,  battery,  known  resistance  and  a  two 
way  switch. 

The  instruments  arc  connected  as  in  fig.  558,  and  in  making  the 
test,  the  voltmeter  is  switched  into  circuit  across  the  known 


484 


HAWKINS  ELECTRICITY 


resistance  and  then  across  the  unknown  resistance,  readings  being 
taken  in  each  case.  The  value  of  the  unknown  resistance,  is  then 
easily  calculated  from  the  following  proportion: 


drop  across  known  resistance 


known  resistance 


drop  across  unknown  resistance      unknown  resistance 


Fig.  559. — Leeds  and  Northrup  portable  galvanometer  (pointer  type  A).  The  sensitiveness  of 
this  instrument  is  such  that  it  may  be  substituted  in  numerous  cases  for  the  non-portable 
reflecting  type  of  galvanometer;  as  for  instance,  in  the  checking  of  ammeters  and  volt- 
meters to  an  accuracy  of  .2%  by  the  potentiometer  method,  and  on  almost  all  Wheatstone 
bridge  measurements  to  commercial  accuracies.  A  current  of  2  micro-amperes  will  cause 
the  pointer  to  move  1  mm.  over  the  scale,  that  is,  it  has  a  sensibility  of  500.000  ohms. 
The  method  of  suspending  the  moving  system  is  such  as  to  practically  eliminate  initial 
friction  which  is  of  value  in  all  zero  deflecting  methods.  The  suspensions  and  moving 
system  are  guarded  by  springs,  which  together  with  the  solid  construction  of  the  case, 
render  the  instrument  capable  of  withstanding  rough  usage.  Overall  dimensions  are 
5^"  X  2fi"  X  3H;  weight  about  3  pounds. 


from  which 

unknown  \      known  resistance  X  drop  across  unknown  resistance 
resistance/  drop  across  known  resistance 


TESTING  AND    TESTING  APPARATUS 


485 


Oues.    What  may  be  substituted  for  the  voltmeter? 

Ans.  A  high  resistance  galvanometer,  whose  deflections  are 
proportional  to  the  current,  the  value  of  the  deflections  being 
substituted  in  the  formula. 

Oues.  What  precaution  should  be  taken  in  making 
the  test? 

Ans.  The  current  used  should  not  be  strong  enough  to 
appreciably  heat  the  resistance,  and  if  the  current  be  not  very 
steady,  several  readings  should  be  taken  of  each  measurement 
and  the  average  values  used  in  the  formula. 


Fig.  500. — Voltmeter  method  of  testing  resistances.  Knowing  the  resistance  of  the  voltmeter, 
turn  switch  to  the  left  and  from  reading  calculate  resistance  corresponding  to  one  division 
of  the  scale.  Turn  switch  to  right  and  multiply  reading  by  resistance  required  for  deflec- 
tion of  one  division  This  gives  resistance  of  voltmeter  and  unknown  resistance;  sub- 
tracting from  this  the  resistance  of  voltmeter  gives  value  of  the  unknown  resistance. 

Oues.     How  are  the  most  accurate  results  obtained? 

Ans.     By  selecting  the  known  resistance  as  near  as  possible 
to  the  supposed  value  of  the  unknown  resistance. 


Voltmeter  Method. — This  is  a  direct  deflection  method  and 
consists  in  determining  first  the  resistance  that  will  deflect  the 
needle  through  one  division  of  the  scale  on  a  given  battery 


486 


HAWKINS  ELECTRICITY 


current,  then  with  this  as  a  basis  for  comparison  the  voltmeter 
is  connected  across  the  unknown  resistance  whose  value  is 
easily  calculated  from  the  reading. 

In  making  the  test,  the  instruments  are  connected  as  in  fig.  560. 
The  current  from  battery  is  first  passed  through  the  galva- 
nometer by  turning  switch  as  shown. 


Pig.  561. — Megohm  box  or  set  of  standard  high  resistances.  The  box  contains  five  resist- 
ances of  200,000  ohms  each.  The  six  pillars  are  petticoat  insiilated,  the  resistances  being 
placed  between  each  pair  of  pillars.  There  is  a  double  contact  post  on  top  of  each  piUai 
so  that  these  can  be  connected  together  with  copper  links. 


Assuming  that  the  resistance  of  the  instrument  is  8,000  ohms  and 
that  the  current  deflects  the  needle  through  10  divisions  of  the  scale, 
then  for  a  deflection  of  one  division  the  resistance  is 

8,000  X  10  =  80,000  ohms. 

Accordingly,  if,  when  the  switch  is  moved  to  the  right,  connecting 
the  voltmeter  across  the  unknown  resistance,  the  needle  be  moved 
through  6  divisions  of  the  scale,  the  combined  resistance  of  the  volt- 
meter and  unknown  resistance  is 

80,000  -^  6  =  13,333i  ohms, 

and  subtracting  the  resistance  of  the  voltmeter,  the  value  of  the  un- 
known resistance  is 

13,333i  -  8,000  =  5,333J  ohms. 


TESTING  AND    TESTING  APPARATUS 


487 


Ques.     For  what  kinds  of  test  is  the  voltmeter  method 
best  adapted  ? 

Ans.     For  measuring  high  resistances,  as  the  insulation   of 
wires,  etc. 

Ques.    What  may  be  said  with  respect  to  the  current 
used? 

Ans.     Its  voltage  should  be  as  high  as  possible  within  the 
limits  of  the  voltmeter  scale. 


Fig.  563.  —  Standard  resistance  box: 
lOO.OOOohms.infour  units  of  10,000, 
20,000,  30,000,  and  40,000  ohms. 
An  "infinity"  plug  separates  each 
coil  from  the  ones  adjacent.  Seg- 
ments are  elevated  from  the  hard 
rubber  top  by  special  washers  in 
order  to  increase  insulation.  Bind- 
ing posts  are  so  arranged  as  not  to 
be  in  the  way  when  plugs  are  used. 

Pig.  562. — Standard  high  resistance  box:  100,000  ohms.  It  is  mounted  in  a  brass  box  with 
a  hard  rubber  top.  Connections  should  be  made  to  terminals  marked  3  and  4.  When 
the  flexible  cord  is  on  plug  1,  the  box  is  short  circuited,  but  when  on  plug  2,  the  resist- 
ance of  100,000  ohms  is  in  series.    The  box  is  especially  suited  to  rapid  cable  testing. 


Ques.     In  testing  cable  insulation  what  is  desirable  with 
respect  to  voltmeter  and  current? 

Ans.     A  low  reading  voltmeter  should  be  used  in  connection 
with  a  large  battery. 


488 


HAWKINS  ELECTRICITY 


Wheatstone  Bridge  Method. — For  accurate  measurements 
of  resistance  this  method  is  almost  universally  used.  The  so- 
called  "Wheatstone"  bridge  was  invented  by  Christie,  and 
improperly  credited  to  "UTieatstone,  who  simply  applied  Christie's 
invention  to  the  measurement  of  resistances. 

The  bridge  consists  of  a  system  of  conductors  as  shown  in 
fig.  564.  The  circuit  of  a  constant  battery  is  made  to  branch  at 
P  into  two  parts,  which  re-unite  at  0,  so  that  part  of  the  cturent 


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Pig.  564. — Diagram  showing  principle  of  Wheatstone's  bridge.  A,  B.  C.  and  D.  are  the  fout 
members  which  constitute  the  bridge.  The  current  from  the  batten,-  dixndes  at  P.  part 
traversing  DC,  and  part  traversing  B.\.  The  galvanometer  connected  to  M  and  X  will 
indicate  when  the  currents  are  equal  in  the  two  branches  by  gi\-ing  no  deflection.  This  is 
then  a  zero  or  nil  method  of  testing.  The  resistances  and  keys  required  in  testing  are  shown 
in  fig.  565.  In  the  actual  instrument,  the  members  A,  B,  C,  and  D  are  known  by  the 
names  given  in  the  figure. 


flows  through  the  point  M,  the  other  part  through  the  point  N. 
The  four  conductors  A,  B,  C,  D,  are  spoken  of  as  the  arws  of 
the  balance  or  bridge.    It  is  by  the  proportion  existing  between 


TESTING  AND    TESTING  APPARATUS 


489 


the  resistances  of  these  arms  that  the  resistance  of  one  of  them 
can  be  calculated  when  the  resistances  of  the  other  three  are 
known.  When  the  current  which  starts  from  the  battery  arrives 
at  P,  the  pressure  will  have  fallen  to  a  certain  value.  The  pressure 
in  the  upper  branch  falls  again  to  M,  and  continues  to  fall  to  Q. 
The  pressure  of  the  lower  branch  falls  to  N,  and  again  falls  till  it 
reaches  the  value  at  Q.     Now  if  N  be  the  same  proportionate 


BATTERY  KE.Y 


Fig.  56.5. — Diagram  showing  arms  of  Wheatstone  bridge,  resistances  and  method  of  connecting 
galvanometer,  battery  and  unknown  resistance. 


distance  along  the  resistances  between  P  and  Q,  as  M  is  along 
the  resistances  of  the  upper  line  between  P  and  Q,  the  pressure 
will  have  fallen  at  N  to  the  same  value  as  it  has  fallen  to  at  M ; 
or,  in  other  words,  if  the  ratio  of  the  resistance  C  to  the  resistance 
D  be  equal  to  the  ratio  between  the  resistance  A  and  the  re- 
sistance B,  then  M  and  N  will  be  at  equal  pressures.  To  find  out 
if  this  condition  obtain,  a  sensitive  galvanometer  is  placed  in  a 


490 


HAWKINS  ELECTRICITY 


branch  wire  between  M  and  N  which  will  show  no  deflection  when 
M  and  N  are  at  equal  pressure  or  when  the  iour  resistances  of  the 
arms  "  balance  "  one  another  by  being  in  proportion,  thus: 


A :C  =  B  :D 


(1) 


KEY 


6MVAN0METEf{ 


•     O     0     0 


N         1,000      100        10  Q        10        100      1000         M 


»  »  » 


INF  I       B         t  2 

0         (!>        0        g- 


m   »    »    »    <>    i) 


P  50  10  ZO  10  10 


UNKNOWM    RtSlSTWK.E 

■<V\AAAAA/vV\A\VvV\AAAAA/*- 


FlG.  566. — Diagram  showing  usual  arrangement  of  resistances  in  arms  of  Wheatstone's  bridge. 
In  practice  the  bridge  is  seldom  or  never  made  in  the  lozenge  shape  of  the  diagrams,  figs. 
564  and  565,  these  being  given  merely  for  clearness.  The  resistance  box  of  fig.  554  is.  in 
itself,  3  complete  "bridge,"  the  appropriate  connections  being  made  by  screws  at  various 
points.  The  letters  in  the  above  diagram  corespond  with  those  in  figs.  564  and  565,  and 
the  three  figures  should  be  carefully  compared. 


If,  then,  the  value  of  A,  B,  and  C  be  known,  D  can  be  calculated. 
The  proportion  (1)  is  reduced  to  the  following  equation  before 
substituting. 


TESTING  AND   TESTING  APPARATUS 


491 


For  instance,  if  A  and  C  be,  as  in  fig.  565,  10  ohms  and  100  ohms 
respectively,  and  B  be  15  ohms,  D  will  be  (15  X  100)  ^  10  =  150 
ohms. 

As  constructed,  Wheatstone  bridges  are  provided  with  some 
resistance  coils  in  the  arms  A  and  C,  as  well  as  with  a  complete 


Pig.  567. — Standard  resistance  box  and  Wheatstone  bridge.  This  pattern  is  a  modification 
of  the  Anthony  form  of  bridge.  All  the  resistances  are  wound  upon  metal  spools.  The 
bridge  ratio  coUs  are  1,  10.  100,  1,000.  10,000.  The  rheostat  coils  are  arranged  in  five 
rows,  of  ten  coils  each.  The  ordinary  decade  plan  (explained  in  fig.  570)  is  followed.  The 
coils  may  be  joined  in  series  in  multiple,  or  in  any  combination  of  series  and  multiple. 
The  coils  may  thus  be  checked  against  each  other  in  many  combinations.  For  instance, 
all  the  ten  ohm  coils  taken  in  parallel  may  be  compared  with  any  one  ohm  coil.  The 
precision  of  adjustment  is  said  to  be  kq%  for  the  coils  of  the  tenth  ohm  series,  and  g^% 
for  the  coils  of  the  rheostat.  The  ratio  coils  are  certified  to  be  like  each  other  to  within 
r^% .  The  box  is  supplied  with  battery  and  galvanometer  keys  of  substantial  construction. 


set  in  the  arm  B.  The  advantage  of  this  arrangement  is  that 
by  adjusting  A  and  C,  the  proportionality  between  B  and  D  can 
be  determined,  and  can,  in  certain  cases,  be  measured  to  fractions 
of  an  ohm.  In  fig.  565  resistances  of  10,  100,  and  1,000  ohms 
are  included  in  the  arms  A  and  C. 


492 


HAWKINS  ELECTRICITY 


Ques.    Describe  the  method  of  testing  with  the  bridge. 

Ans.  Fig.  567  illustrates  the  general  arrangement  of  re- 
sistances to  be  found  in  an  ordinary  bridge.  The  connections 
are  made  as  shown.  In  testing,  first  depress  the  battery  key, 
then  tap  the  galvanometer  key.  This  should  be  repeated  ad- 
justing the  resistances  till  no  deflection  is  obtained.     The  re- 


lOCX)       ICO 


lOOO 


Fig.  56S. — Ratio  coils  of  Wheatstone  bridge.  Almost  every  box  intended  to  serve  as  a  Wheat- 
stone  bridge  is  furnished  with  a  set  of  coils  which  forms  the  arms  of  proportion  or  ratio 
arms  of  the  bridge.  There  is  a  choice  of  several  different  ways  of  arranging  these  coils. 
The  figure  shows  the  simplest  arrangement,  which  is  employed  in  bo.xes  not  intended  for  tlit 
hiehest  accuracy.  The  required  ratio,  as  for  instance  1:100,  is  obtained  by  withdrawir.e  a 
,        ,  ,  ,        J  r>      r>    .•        1     1        1         10       ,  1.000    1.000   1.000    100 

plug  from  each  arm  A  and  B.  Ratios  pyjj. —.  j^.  etc.,  or  "y  • -^q" '"loo  '  Too'^"^"^'^ 
obtainable  in  this  manner.  This  simple  arrangement  is  oi)en  not  only  to  the  objection 
that  the  contact  resistance  of  the  plugs  which  remain  in  is  always  included  with  the  re- 
sistance unplugged,  but  also  to  all  other  objections  to  be  urged  against  the  use  of  many 
plugs  where  a  few  will  do.  The  method  has  the  limitation  that  it  is  not  possible  to  reverse 
the  arms  of  the  bridge,  that  is,  to  transpose  the  arms  A  and  B. 


sistance  then  in  the  arm  B    X    (C 
of  the  unknown  resistance. 


A)  will  give  the   value 


Ques.     Why  should  the  battery  key  be  depressed  before 
the  galvanometer  key. 

Ans.     To  avoid  the  sudden  swing  of  the  galvanometer  needle, 
which  occurs  on  closing  circuit  in  consequence  of  self-induction. 


TESTING  AND   TESTING  APPARATUS 


493 


Ques.  How  is  it  known  whether  too  much  or  too  little 
resistance  be  unplugged  ? 

Ans.  The  galvanometer  needle  will  be  deflected  to  one  side 
for  too  much  resistance,  and  to  the  opposite  side  for  too  little 
i-esistance. 


Fig.  569. — Method  of  reversing  arms  of  Wheatstone  bridge  with  reversing  blocks.  The  arrange- 
ment shown  in  the  figure  is  •classical,  being  that  used  in  the  English  post  office  type  of 
Wheatstone  bridge.  It  is  open  to  the  objections  which  apply  to  the  use  of  several  plugs, 
one  of  which  is  withdrawn  to  obtain  the  desired  resistance. 


Ques.  What  is  the  meaning  of  "  Inf.,"  marked  on  the 
bridge? 

Ans.  It  stands  for  "infinity,"  because  the  resistance  coil  at 
the  point  marked  infinity  is  omitted  so  that  adjacent  sections  of 
the  arm  are  disconnected  when  the  plug  is  taken  out. 


In  fact,  the  air  gap  interposed  by  the  removal  of  tlie  plug  by  no  means 
provides  an  infinitely  great  resistance,  but  is  usually  called  such  because 
it  is  vastly  greater  than  any  of  the  other  resistances  of  the  bridge. 


494 


HAWKINS  ELECTRICITY 


*  Figs.  570  and  571. — Dia- 
grams illustrating  the 
decade  plan  of  combin- 
ing resistance  coils.  In 
this  method  the  coils  are 
connected  in  series  and 
the  arrangement  avoids 
the  disadvantage  of  the 
ordinary  Wheatstone  bridge  in  that  the  lat- 
ter requires  a  large  number  of  plugs  to  short 
circuit  the  resistances  not  in  use,  which 
introduces  an  element  of  uncertainty  as  to 
resistance  of  the  plug  contacts  and  the 
necessity  of  adding  up  the  values  of  all  the 
unplugged  resistances  in  order  to  deter- 
mine the  total  resistance  in  circuit.  The 
necessary  regular  succession  of  values  in  a 
rheostat  built  on  the  decade  plan  can  be 
obtained  with  either  nine  or  ten  coils  per 
decade.  The  chief  reason  for  using  the 
latter  number  is  found  in  the  facility  with 
which  all  the  coils  of  one  decade  can  be 
compared  with  one  coil  of  the  next 
higher  decade,  thus  permitting  the  coils 
of  a  rheostat  to  be  checked  among  them- 
selves. Thus,  the  ten  1  ohm  coils  can  be 
checked  with  a  10  ohm,  the  ten  lO's  with  a 
100,  etc.  In  some  sets  the  ten  coils  of  a 
a  decade  can  be  connected  in  series  or  in 
parallel,  and  it  then  becomes  an  advantage  to  have  ten  coils  to  a  decade,  since  the  coils  in 
one  decade  in  parallel  equal  one  of  the  coils  of  the  next  lower  decade.  When  these  latter 
advantages  are  not  required,  and  especially  when  dials  or  sliding  switches  are  used,  there 
is  Uttle  or  no  advantage  in  using  more  than  nine  coUs  per  decade,  as  shown  in  fig.  570. 
Here  all  the  coils  of  the  set  are  cormected  in  series  so  that  the  circuit  is  never  open.    Thus  it 


imnrlnnriUrij 

/O         /O         M         lO  /O         /O  /O         lO         /O 


is  a  slight  advantage  to  have  permanent  connections  a,  6,  and  c,  because  all  the  coils  of  a 
decade  can  be  thrown  in  circuit  by  simply  pulling  out  a  plug,  it  not  being  necessary  to 
insert  it  again,  as  would  be  the  case  if  the  a,  ft,  and  c  connections  were  not  used.  More- 
over, if  any  plug  make  bad  contact,  its  eflfect  is  somewhat  lessened  by  having  this  bad 
contact  shunted  by  the  remaining  coils  of  the  decade.  Again,  there  are  occasions  where 
\-iolent  deflections  of  a  galvanometer  are  prevented  by  not  having  the  circuit  entirely 
oi>en  when  a  plug  is  taken  out. 


TESTING  AND   TESTING  APPARATUS 


495 


The  Decade  Plan. — In  this  method  of  combining  resistance 
coils,  there  are  9  or  10  one  ohm  coils  for  the  units  place,  9  orlO 
ten  ohm  coils  for  the  tens  place,  9  or  10  one  hundred  ohm  coils 
for  the  hundreds  place  and  so  on.  Each  series  of  coils  of  the 
same  value  is  designated  a  decade.  The  connections  are  usually 
made  as  shown  in  figs.  570  and  571. 

It  is  apparent  from  the  figure  that  any  value  in  any  one  decade  can 
be  obtained  by  inserting  between  a  bar  and  a  block,  only  one  plug; 
moreover  if  several  decades  be  in  series,  any  value  up  to  the  limit  of  the 
set  can  be  read  off  directly  from  the  position  of  the  plugs  without  having 
to  add  up  the  unplugged  resistance  as  in  the  ordinary  arrangement. 


Fig.  572 — Two  plug  arrangement  of  ratio  coils.  Each  of  the  ratio  coils  has  one  of  its  terminals 
connected  to  a  common  center  which  corresponds  to  the  block  marked  C  in  the  figure.  The 
other  terminal  of  each  coil  is  connected  to  an  individual  block,  there  being  one  block  for 
each  coil.  The  bar  B  on  one  side  of  these  blocks  is  joined  to  the  rheostat  and  the  bar  A 
on  the  other  side  to  an  X  post.  In  the  ordinary  use  of  this  set  of  ratio  coils  two  plugs  only 
are  used.  One  plug  is  inserted  between  the  bar  A  and  one  of  the  blocks,  1,  1',  10,  10',  etc., 
of  the  central  row  of  blocks.  The  other  plug  is  inserted  between  the  bar  B  and  any  one 
of  the  other  blocks  of  the  central  row.  There  are  two  ratio  coils  of  each  value.  To  obtain 
an  even  ratio  as  1,000  to  1,000',  one  plug  is  inserted  between  the  block  1,000  and  the  bar 
A,  and  the  other  plug  between  the  1,000  block  and  bar  B,  the  ratio  arms  are  reversed; 
that  is,  the  1,000  ohm  coil  is  connected  to  the  X  post  and  the  1,000  ohm  coil  to  the  end  of 
the  rheostat.  When  uneven  ratios  are  used,  the  same  ratio  can  be  obtained  by  four  different 
combinations.  To  obtain  the  ratio  one  to  ten,  insert  a  plug  between  A  and  1,  and  another 
between  B  and  10,  or  between  A  and  1',  and  B  and  10,  and  get  1:10,  or  between  A  and  1, 
and  B  and  10',  and  get  1 :10',  or  again,  between  A  and  1'  and  B  and  10',  and  get  1'  to  10'. 
Other  ratios  are  obtained  in  a  similar  manner.  By  using  more  than  two  plugs  and 
connecting  certain  of  the  coils  in  parallel  combinations,  a  large  number  of  other  ratios 
may  be  obtained.  This  arrangement  offers  a  convenient  method  of  measuring  the  sensi- 
bility of  a  bridge  and  galvanometer  combination  that  is  frequently  applicable.  If  for 
instance  the  one  ohm  coil  is  used  on  either  side  after  a  balance  has  been  obtained  the  one 
ohm  may  be  shunted  with  the  1,000  ohm  on  the  same  side.    This  will  make  a  variation  of 

—  of  1%  and  the  galvanometer  deflection  may  be  noted  for  this  variation.  Similarly, 
the  1  ohm  may  be  shunted  with  the  100  for  a  variation  of  1%,  or  with  the  10,000  for  a 
variation  of  rrr^  of  1%.  The  ten  ohm  coil  may  be  shunted  with  the  1,000  for  a  variation 
of  1%  and  with  the  10,000  for  a  variation  of  tt;  of  1%.  In  the  arrangement  of  ratio  coils, 
errors  due  to  plug  contacts  are  negligible  because  only  two  plug  contacts  enter  the  circuit, 
and  with  an  even  ratio,  it  is  only  the  difference  in  the  resistances  of  the  two  plug  contacts 
that  can  affect  the  result.  In  measuring  any  of  the  ratio  coils  while  in  the  bo.x  it  is  only 
necessary  to  connect  to  the  bar  C  and  to  either  the  bar  A  or  B  and  plug  in  the  coils  to  be 
measured. 


496 


HAWKINS  ELECTRICITY 


/VWWWWVNf —     -f- 


{^ 


3' 


A^AAA/vA/vVv/J 


Of)- 


<(^ 


lAAAA/VWVNAA^ 

(-5) 


Figs.  573  and  574. — The  Leeds  and 
Northrup  decade.  The  object  of 
this  arrangement  is  to  reduce  the 
number  of  coils  required.  In  fig. 
573,  the  1,  3',  3  and  2  are  con- 
nected in  series.  Let  the  ter- 
minals of  the  1  ohm  and  2  ohm 
coils  be  numbered   (1),  (2),  (3), 

(4)  and  (5)  (fig.  573).  The  cur- 
rent enters  at  point  (1)  and 
leaves  the  coils  at  the  point  (5), 
traversing  1,  3',  3,  2  =9  ohms  in 
all.  If  this  series  be  multiplied 
by  any  factor  n,  then  n  ( 1  +3'  -H 
3+2)  =  n  9  ohms.  It  will  be 
seen  that  if   the   points  (I)  and 

(5)  be  connected,  all  the  coils 
are  short  circuited,  and  the  cur- 
rent will  traverse  zero  resistance. 
If  the  points  (2)  and  (5)  be  con- 
nected, the  3',  3  and  2  ohm  coils 
will  be  short  circuited  and  the 
current  will  traverse  1  ohm.  By 
extending  the  process  so  as  to 
connect  two  and  only  two  points 
at  a  time  it  is  possible  to  obtain 
the  regular  succession  of  values  n 
(0,  1,2.  3,  4.5.  6,  7, 8,  9).  the  last 
being  obtained  when  no  points 
are  connected.  Fig.  574  shows 
Leeds  and  Northrup's  method  of 
connecting  these  points  two  at  a 
time  \vith  the  use  of  a  single  plug. 
The  circles  in  the  diagram  repre- 
sent two  rows  of  ten  brass  blocks 
each.  To  the  first  two  blocks  at 
the  top  of  the  rows,  the  points  (5) 
and  (1),  fig.  573,  are  connected; 
to  the  second  two,  the  points  (2) 
and  (5)  are  connected,  etc.,  no 
points  being  connected  to  the  last 
pair  of  blocks.  Hence,  if  a  plug 
be  inserted  between  blocks  1  and 
5,  fig.  575,  the  points  (l)and  (5)  of 
diagram  fig.  573  are  connected, 
giving  the  value  of  0,  if  between 
the  blocks  2  and  5  the  points  (2) 
and  (5)  are  connected,  giving  the 
value  1,  and  so  on.  The  value 
9  is  obtainable  when  the  plug  is 
in  the  last  pair  of  blocks,  which 
have  no  connections.  Fig.  572 
shows  a  top  view  of  the  blocks 
of  a  simple  decade  constructed 
upon  this  plan. 


o    o 


TESTING  AND    TESTING  APPARATUS  497 

Oues.  What  other  advantages  are  gained  with  the 
decade  arrangement? 

Ans.  The  single  plug  used  with  each  decade  is  never  out  of 
use,  being  either  in  the  zero  position  or  set  on  some  value,  and  is 
therefore  not  easily  lost  by  being  laid  aside.  The  use  of  only  one 
plug  in  a  decade  makes  it  easy  to  ascertain  that  the  plug  is 


Fig.  575. — Leeds  and  Northrup  dial  Wheatstone  bridge.  Rotating  switches  are  used  instead  of 
plugs,  which  permits  quicker  adjustment  of  the  resistances,  adapting  it  to  rapid  working. 
The  ratio  coils  are  arranged  as  in  fig.  568.  There  are  four  dials  which  form  the  rheostat. 
The  units  dial  contains  9  one  ohm  coils;  the  tens  dial,  9  ten  ohm  coils;  the  hundreds  dial, 
9  one  hundred  ohm  coils,  and  the  thousands  dial  9  one  thousand  ohm  coils.  The  values 
of  the  ratio  coils  are  1,  1.  10,  10,  100,  100,  1,000,  1,000,  10,000,  10,000. 

making  good  contact  as  only  one  block  in  a  row  is  plugged  at  a 
time,  the  other  blocks  are  not  kept  under  a  strain  by  having 
plugs  forced  tightly  between  them. 

This  strain  on  the  blocks,  which  always  exists  in  those  sets  in  which 
a  resistance  is  thrown  in  by  removing  a  plug,  tends  to  separate  or  loosen 
them  and  often  to  warp  the  hard  rubber  upon  which  they  are  mounted. 
Another  advantage  of  the  decade  plan  is  that  it  permits  obtaining  a 
succession  of  values  by  means  of  sliding  contacts  or  dial  switches,  a 
method  which  is  becoming  deservedly  more  appreciated. 


498 


HAWKINS  ELECTRICITY 


Oues.  What  is  the  difiference  between  "  plug  out  "  and 
*'  plug  in  "  types  of  resistance  box? 

Ans.  In  the  plug  out  type,  resistance  is  put  in  the  circuit 
by  remo\'ing  plugs,  as  in  fig.  565;  in  the  plug  in  type,  resistance 
is  put  in  the  circuit  by  inserting  plugs  as  in  figs.  570  and  571. 


Fig.  576. — Queen  Acme  portable  testing  set.  It  consists  of  a  Wheatstone  bridge,  with  reversi- 
ble arms,  battery  of  four  dry  cells,  D'Arsonval  galvanometer,  batter>'  and  galvanometer 
keys.  There  ^re  sixteen  resistance  coils,  having  a  combined  resistance  of  11,110  ohms. 
Each  bridge  arm  is  provided  w-ith  three  coils  of  1,  10,  100  ohms,  and  10,  100,  1.000  ohms 
respyectively.  The  commutator  admits  of  a  ratiool  1  to  l.OOOon  either  bridge  arm, gi\-ing 
the  set  a  theoretical  range  from  .001  of  an  ohm  to  11.110.000  ohms.  For  resistances  above 
1,000.000  ohms,  the  normal  battery  power  must  be  increased.  The  contact  keys  are 
located  as  shown.  The  battery  key  has  single  contact,  but  the  galvanometer  key  has 
double  contact ;  depressing  it  closes  the  galvanometer  circuit,  and  releasing  it  short  circuits 
the  galvanometer,  bringing  the  latter  quickly  to  rest. 


TESTING  AND    TESTING  APPARATUS 


499 


Testing  Sets. — For  convenience  in  testing,  a  combination 
of  the  instruments  used  is  put  up  in  a  neat  and  substantial  case, 
and  known  as  a  testing  set.  There  are  innumerable  forms  of 
testing  set,  a  few  of  which  are  shown  in  the  accompanying  illus- 
trations. The  usual  combination  is  a  Wheatstone  bridge,  galva- 
nometer, battery  and  necessary  keys  and  connections. 


Fig. 


577. — Connections  and  circuits  of  Queen  acme  portable  testing  set.  There  are  three 
rows  of  blocks,  LL',  MM',  NN.  LL  is  connected  to  NN'  by  means  of  a  heavy  copper 
bar,  joining  L'  and  N'.  LL'  and  NN'  constitute  the  rheostat,  from  which  any  resistance 
from  1  ohm  to  11,110  ohms  may  be  obtained  by  removing  the  proper  plugs.  The  block  N 
of  the  rheostat  is  connected  to  the  lower  line  post  D.  The  upper  line  post  C  is  connected 
to  the  block  X  of  the  commutator.  The  block  C  has  no  other  permanent  connection, 
except  key  G.  The  block  R  of  the  commutator  is  connected  to  the  block  L  of  the  rheostat, 
and  has  no  other  connection,  excepting  by  plugs.  Each  half  of  MM' constitutes  a  bridge  arm, 
designated  A  and  B  respectively.  Beginning  at  the  lower  line  post  D,  the  cornections 
form  a  continuous  circuit  through  the  rheostat,  thence  through  the  bridge  arm  B,  thence 
through  the  bridge  arm  A,  thence  to  the  upper  line  post  C.  The  commutator  serves 
merely  to  reverse  the  bridge  arms  A  and  B.  The  battery  terminals  are  connected  as 
shown:  the  positive  terminal  directly  to  the  common  junction  of  the  two  bridge  arms, 
and  the  negative  terminal  through  the  battery  key  to  the  rheostat.  The  positive  terminal 
of  the  galvanometer  is  connected  through  the  galvanometer  key  with  the  block  X,  and 
the  negative  terminal  with  the  block  R  of  the  commutator,  or.  what  is  equivalent,  with 
the  block  L  of  the  rheostat.  The  commutator  blocks  A  B,  R  and  X,  are  connected  by 
plugs  as  shown.  When  the  commutator  plugs  are  in  the  position  PQ,  the  bridge  arm  B 
is  connected  to  the  rheostat  and  the  bridge  arm  A  is  connected  to  the  line,  the  ratio 
between  the  bridge  arms  ratio  being  A  -^  B  =  X  -^  R  but  when  the  plugs  are  in  the 
position  ST,  the  bridge  arms  are  reversed  in  position  A.  being  connected  with  the  rheostat 
and  B,  with  the  line,  and  the  bridge  arm  ratio  becomes  A  -^  B  =  R  -^  X.  The  connections 
of  the  testing  set  may  be  more  readily  understood  from  the  simplified  diagram  fig.  578. 


500 


HAWKINS  ELECTRICITY 


Ques.  Describe  the  operation  of  the  Queen  Acme  test- 
ing set  figs.  576  and  577,  in  measuring  resistance. 

Ans.  Connect  the  terminals  of  the  resistance  to  be  measured 
to  the  line  posts  C  and  D.  Place  the  battery  connections  on 
the  two  upper  tips  0  and  1 ,  thus  thro\Ying  one  end  of  the  battery 
into  circuit,  which  is  sufficient  until  an  approximate  balance  is 
obtained.     Employ  the  100  ohm  coil  in  each  bridge  arm,  and 


Fig.  578. — Simplified  diagram  showing  connections  of  Queen  Acme  portable  testing  set. 


place  the  commutator  plugs  in  the  position  PQ,  or  in  the  position 
ST.  Then  remove  plugs  from  the  rheostat  until  the  value  of 
total  resistance  employed,  or  nearly  as  may  be  guessed  is  equal 
to  that  of  the  unknown  resistance.  Now  press  the  battery  key 
Ba,  and  holding  it  down  momentarily,  press  the  galvanometer 
key  Ga.  If  the  galvanometer  needle  swing  to  the  right  toward 
the  symbol  +  the  resistance  employed  in  the  rheostat  is  too 
high  and  must  be  reduced.  If  the  needle  swdng  to  the  left 
toward—,  the  resistance  employed  is  too  low  and  must  be  in- 
creased.   By  altering  the  resistance  of  the  rheostat  accordingly, 


TESTING  AND   TESTING  APPARATUS 


501 


a  value  will  soon  be  found,  which  when  varied  slightly  either 
way,  will  reverse  the  deflection  of  the  galvanometer  needle. 
Now  remove  the  battery  connection  from  tip  1,  and  place  it  on 
the  tip  4,  thus  throwing  the  whole  battery  into  circuit.  Then 
press  the  keys  again  as  before,  first  the  battery  key,  then  the 
galvanometer  key.  This  will  increase  the  deflection  of  the  gal- 
vanometer needle  for  the  same  variation  in  the  rheostat,  thus 


Fig.  579. — Diagram  of  the  Queen  dial  decade  portable  testing  set.  Its  dimensions  are  9H" 
long,  7"  wide,  and  7"  deep,  and  weighs  11  J^o  pounds.  The  resistances  are  arranged  upon  the 
dial  decade  plan,  being  placed  in  circuit  by  means  of  a  rotating  switch  contact.  The 
switches  are  so  constructed  that  they  maybe  turned  in  either  direction,  thereby  permit- 
ting them  to  be  turned  quickly  from  the  highest  resistance  in  any  dial  to  the  lowest 
resistance  in  the  same  dial.  This  arrangement  avoids  the  necessity  of  turning  back 
through  all  the  remaining  resistances  in  any  particular  group  of  coils  and  is  of  value  in 
locating  swinging  crosses  or  conditions  of  momentary  balances.  The  connections  for  the 
various  tests  are  made  by  the  manipulation  of  one  small  knife  switch  (W.B.  —  M.L.)  and 
the  switch  'BA..;  these  are  plainly  lettered,  thus  avoiding  the  necessity  of  referring  to  a 
diagram  of  connections.  In  construction ,  the  dial  switches  are  made  up  of  eight  laminations 
of  No.  28  B.  &  S.  phosphor  bronze,  and  the  form  is  such  as  to  prevent  wearing  grooves 
on  the  top  of  the  contact  studs.  In  this  instrument  the  electrical  circuits  are  soldered 
throughout  excepting  the  switch  contact  whose  resistance  is  negligible.  The  resistances 
are  wound  with  manganin.  The  battery  comprises  six  cells  sub-divided  which  are  easily 
replaceable.  The  galvanometer  is  the  same  as  in  the  Queen  acme  set,  but  has  the  addition 
of  an  Ayrton  shunt,  which  is  useful  in  making  insulation  measurements.  The  necessary 
keys,  binding  posts,  and  switches  are  provided  so  as  to  facilitate  the  use  of  the  instrument 
for  the  various  measurements  that  can  be  made  with  it. 


502 


HAWKINS  ELECTRICITY 


enabling  the  making  of  a  more  accurate  adjustment.  The 
measurement  thus  made  will  be  the  best  result  that  can  be 
obtained  with  bridge  arms  of  equal  value,  but  by  selecting  more 
suitable  values  of  the  two  arms  from  the  following  table  of 
bridge  ratios  a  much  higher  degree  of  accuracy  may  be  obtained. 


Table  Showing  the  Best  Values  of  Bridge  .\nns  for  Measuring  any 
Desired  Resistance 


Value  of  Resistance  being  measured 

Best  values  of 

Position  of 

Commutator 

Plugs  as 

shown  in 

fig.  582 

A  = 

B  = 

Below  1  5  ohms 

1         1.000 

PQ 
PQ 

PQ 
PQ 

PQ  or  ST 
ST 
ST 
ST 

Between  1  5  and  1 1  ohms 

1 

10 

100 

100 

1,000 

1,000 

1,000 

100 
100 
1,000 
100 
100 
10 

1 

"         11  and  78  ohms 

78  and  1,100  ohms 

1,100  and  6,100  ohms 

6,100  and  110,000  ohms 

110,000  and  1,110,000  ohms. 
"     1,110,000  and  11,110,000  ohms 

Oues.  In  testing  with  the  Queen  Acme  set  how  should 
the  plugs  be  placed  in  the  commutator? 

Ans.  iVlways  make  the  arm  A  the  smaller  except  when  the 
two  arms  are  of  equal  value. 

Oues.  If  the  resistance  being  measured  is  higher  than 
6,100  ohms,  or  lower  than  1,100  ohms,  how  should  the  com- 
mutator plugs  be  placed? 

Ans.  If  higher  than  6,100  ohms,  they  should  be  placed  in 
the  position  ST;  if  lower  than  1,100  ohms,  in  position  PQ. 


TESTING  AND    TESTING  APPARATUS 


503 


When  the  plugs  are  placed  in  the  ST  position,  the  unknown  resistance 
is  found  by  dividing  the  value  of  the  larger  bridge  arm  by  that  of  the 
smaller,  and  multiplying  the  total  employed  resistance  in  the  rheostat 
by  the  quotient.  When  the  plugs  are  placed  in  the  PQ  position,  the 
employed  resistance  in  the  rheostat  is  divided  by  the  quotient. 


Fig.  580. — Queen  portable  silver  chloride  testing  battery, 
advantage  of  long  life,  light  weight,  and  compactness, 
new  is  .8  volt. 


The  silver  chloride  cell  has  the 
The  pressure  of  each  cell  when 


Direct  Deflection  Method  with  Queen  Acme  Set. — To 

measure  for  instance,  insulation  resistance  by  direct  deflection 
connect  a  known  high  resistance,  say  100,000  ohms  between  the 
line  post  C  (fig.  577),  and  the  positive  battery  post.  Remove 
all  plugs  from  the  commutator,  and  place  all  plugs  in  the  rheo- 
stat, as  any  employed  resistance  in  the  rheostat  will  be  in  circuit 


504 


HAWKINS  ELECTRICITY 


with  the  galvanometer  and  the  battery.  Place  the  battery 
connection  so  as  to  throw  only  one  cell  into  circuit.  Now  press 
the  keys  and  obtain  a  deflection  of  the  galvanometer  needle. 
For  example:  asstmie  that  the  needle  to  be  deflected  about  8 
di\'isions  of  the  scale.  Since  this  deflection  is  due  to  the  current 
from  one  cell  passing  through  a  resistance  of  100,000  ohms,  then 


Fig.  581. — Ohmmeter.  It  consists  essentially  of  a  slide  wire  Wheatstone  bridge,  with  the  scale 
di%4ded  to  read  either  directly  in  ohms,  or  in  per  cent,  of  a  fixed  resistance  value.  A  gal- 
vanometer is  mounted  on  the  containing  case  of  each,  and  battery  and  galvanometer  keys 
are  pro\nded.  In  the  direct  reading  type,  the  scale  is  so  cut  that  when  the  galvanometer 
is  balanced,  the  pointer  of  the  instrument  indicates  the  value  of  the  resistance  between  the 
X  posts.  The  scale  is  calibrated  for  any  desired  range.  These  ohmmeters  being  slide  wire 
bridges,  the  greatest  accuracy  is  at  the  center  of  the  scale,  hence  one  should  be  selected 
that  will  bring  the  part  of  the  scale  likely  to  be  the  most  used  at  or  near  the  center.  A  con- 
venient tvi)e  is  that  in  which  the  scale  is  cut  in  per  cent.,  100  per  cent,  being  at  the  center 
of  the  scale.  Fixt-d  coils  of  1,  10,  100,  1,000  and  10,000  ohms  are  contained  in  the  instru- 
ment with  apluggingarrangement  allowing  any  one  to  be  used.  When  a  balance  is  obtained, 
the  actual  resistance  is  determined  by  mu!tipl>-ing  the  dial  reading  by  the  value  of  the 
fixed  coil  in  use.  This  amounts  simply  to  shifting  the  decimal  point.  For  instance,  if  the 
100  ohm  coil  were  being  used,  and  the  pointer  were  at  .875,  the  resistance  would  be  87.5 
ohms. 

100,000  X  8  =  .8  megohms  represents  the  resistance  through 
which  one  cell  will  produce  a  deflection  of  one  division  on  the 
scale.    Hence,  .8  megohms  is  the  constant  of  the  galvanometer. 


TESTING  AND   TESTING  APPARATUS 


505 


Now,  replace  the  known  high  resistance  (100,000  ohms)  by 
the  unknown  resistance  (for  instance  such  as  a  cable)  the  value  of 
which  is  to  be  determined.  Add  enough  cells  to  produce  as  large 
a  deflection  of  the  needle  as  possible.  Assume  that  75  cells  give 
a  deflection  of  1.5  scale  division.  Then,  the  galvanometer  con- 
stant multiplied  by  the  number  of  cells  and  the  product  divided 
by  the  deflection  will  give  the  insulation  resistance  of  the  cable ;  or 


Fig.  582. — Commutator  plug  setting  for  comparing  electromotive  forces  by  the  fall  of  potential 
method  with  Queen  acme  set. 

.8  megohm  X  75  cells  =  60.0;   and 
60.0  -T-  1.5  =  40  megohms 

as  the  resistance  of  the  cable. 


Fall  of  Potential  Method  with  Queen  Acme  Set. — To 

compare  electromotive  forces  by  this  method,  place  the  battery 
connection  (fig.  577),  so  as  to  throw  into  circuit  all  the  cells, 
taking  care  not  to  reverse  them  by  crossing  the  battery  cords. 
Plug  the  commutator  as  shown  in  fig.  582,  and  remove  1,000 
ohms  from  bridge  arm  B.  Place  all  plugs  in  arm  A.  From  the 
rheostat  unplug  5,000  ohms.  Then  connect  one  of  the  cells 
being  tested,  with  its  positive  terminal  to  the  +  battery  post 
and  its  negative  terminal  to  the  line  post  C. 


506 


HAWKINS  ELECTRICITY 


When  the  keys  are  pressed,  the  galvanometer  needle  will 
swing  either  to  the  right  or  to  the  left.  If  it  swing  toward  +, 
reduce  the  resistance  in  the  rheostat ;  if  it  swing  toward  — ,  add 
resistance  to  the  rheostat.  When  a  value  is  found  wherein  a 
variation  of  an  ohm  either  way  reverses  the  deflection,  add  to 
this  value  the  resistance  unplugged  in  arm  B,  and  divide  the 
sum  by  the  resistance  in  arm  B.     The  result  gives  the  ratio 


Pig.  583 — Diagram  of  apparatus  for  measuring  low  resistances  based  on  the  principle  of  the 
Kelvin  double  bridge.  In  the  diagram  AB  represents  a  heavy  piece  of  resistance  metal  of 
uniform  cross  section  and  uniform  resistance  per  unit  of  length;  CD  is  another  piece  of 
resistance  metal  of  smaller  cross  section,  and  the  two  are  joined  together  by  a  heavy 
copper  bar,  AC,  into  which  both  are  silver  soldered;  LL  are  the  current  terminals  and  PP 
are  the  pressure  terminals.  The  resistance  of  AB  between  the  marks  0  and  100  on  the 
scale  S  is  .001  ohm.  From  the  point  1  on  the  resistance  CD  to  0  on  AB  is  also  .001  ohm, 
from  2  to  0  is  .002  and  so  on,  and  from  9  to  100  is  .01  ohm.  The  slider  M  moves  along  the 
resistance  AB  and  its  position  is  read  on  the  scale  S  which  is  divided  into  100  equal  parts 
and  can  be  read  by  a  vernier  to  thousandths.  Subdivided  in  this  way  the  resistance  between 
the  tap  off  points  PP  may  have  any  value  from  .001  to  .01  ohms  by  steps  of  .000001  ohm. 


between  the  voltages  of  the  testing  set  battery  and  cell  being 
tested  respectively.  The  division  is  decimal  and  may  be  readily 
accomplished  by  merely  pointing  off  as  many  places  as  there  are 
ciphers  in  the  resistance  employed  from  arm  B.  This  operation 
repeated  with  any  number  of  different  cells,  will  give  their 
voltages  in  terms  of  the  voltage  of  the  testing  set  battery,  and 
from  these  ratios  their  relative  values  may  be  readily  obtained. 


TESTING  AND   TESTING  APPARATUS 


507 


If  the  testing  set  battery  be  replaced  by  a  standard  cell,  the 
first  measurement  gives  at  once  the  voltage  of  the  cell  tested. 

If  the  voltage  of  the  cell  or  battery  being  tested  exceed  that 
of  the  testing  set  battery,  reverse  the  position  of  the  two  batteries, 
and  the  subsequent  operations,  as  outHned  above,  will  give  the 
desired  results. 

How  to  check  a  Voltmeter  with  the  Queen  Acme  Set.— 

In  using  a  set  as  in  fig.  576,  first  remove  about  10,000  ohms  from 
the  rheostat,  plug  the  commutator  as  shown  in  fig.  582,  remove 
100  ohms  from  the  arm  B,  of  the  bridge,  and  connect  a  standard 


Fig.  5g4.— Kelvin  bridge.  This  includes  a  low  resistance  standard  of  .1  ohm  variable  by  steps 
of  .00001  ohm,  a  set  of  ratio  coils,  and  a  holder  for  rods  or  wires  to  be  measured,  with  a 
scale  to  measure  their  length.  It  is  also  provided  with  heavy  flexibles  to  be  used  in 
measuring  the  resistances  of  irregularly  shaped  pieces.  The  connections  are  clearly  shown 
in  the  diagram.  The  range  of  measurements  of  this  bridge  is  :  1  ohm  to  .1  ohm  hv  steps 
of  .001  ohm  readily  estimated  to  .0001;  .1  to  .01  ohm  by  steps  of  .0001  ohm  readily  es- 
timated to  .00001;  .01  ohm  to  .001  ohm  by  steps  of  .00001  ohm,  readily  estimated 
to  .000001;  .001  ohm  down  by  steps  of  .00001  ohm,  readily  estimated  to  .000001  ohm. 


cell  with  the  positive  terminal  to  the  +  battery  post  and  the 
negative  terminal  to  the  line  post  C.  Then,  connect  the  circuit 
to  the  battery  posts  of  the  testing  set  the  positive  lead  to  the 
+  post  and  the  negative  lead  to  the  —  post.  Now,  press  both 
keys  and  note  the  direction  of  the  deflection  of  the  galvanometer 
needle.  If  it  move  toward  + ,  the  rheostat  resistance  is  too  high; 
if  toward  — ,  too  low. 


508 


HAWKINS  ELECTRICITY 


Change  the  rheostat  resistance  accordingly  until  the  balance 
attained  is  su  :h  that  a  very  slight  variation  of  the  rheostat  re- 
sistance one  way  or  the  other  will  reverse  the  galvanometer  de- 
flection. To  find  the  pressure  on  the  circuit,  add  100  to  rheostat 
resistance  and  point  off  two  places.  Multiply  this  value  by  the 
voltage  and  the  product  will  be  the  desired  voltage. 

If  the  voltage  of  the  standard  cell  be  exactly  one  volt,  the 
total  employed  resistance  represents  the  ^•ni^aje  on  the  circuit. 


P^G.  585. — Oueen  slide  wire  bridge.  It  consists  of  a  portable  slide  wire,  Wheatstone  bridge 
arranged  to  read  directly  in  ohms  in  addition  to  its  use  for  locating  crosses  and  grounds. 
It  is  complete  with  battery,  galvanometer  and  telephone  receiver.  The  bridge  is  balanced 
by  moving  the  hand  stylus  until  the  galvanometer  shows  no  deflection  oruntil  there  is  no  sound 
in  the  telephone  receiver.  In  order  to  provide  a  wide  range  of  measurement  and  maximum 
accuracy,  ratio  coils  or  multipliers  ha\'ing  values  of  1.  10,  100,  1,000  and  10.000  are  pro- 
vided. The  scale  of  the  instrument  is  arranged  in  two  parts,  one  of  which  indicates  ohms 
and  the  other  is  divided  into  uniform  di\'isions  for  use  when  locating  crosses  and  grounds 
by  the  Murray  and  Varley  loop  methods.  A  small  induction  coil  is  included  so  as  to  furnish 
an  alternating  current  when  using  the  telephone  receiver. 

For  instance,  in  making  a  measurement  on  a  110  volt  circtiit,  assume 
that  the  emplo3ang  of  7,840  ohms  rheostat  resistance  produces  balance, 
and  that  increasing  or  decreasing  this  resistance  by  two  ohms,  reverses 
the  galvanometer  deflection.  This  indication  that  the  setting  7,840  is 
uncertain,  about  ^  of  1  per  cent.  Since  the  rheostat  coils  are  ad- 
justed to  an  accuracy  of  only  |  of  1  per  cent.,  that  will  be  about 
the  accuracy  of  the  measurement. 


TESTING  AND    TESTING  APPARATUS 


509 


If  the  pressure  of  the  standard  cell  be  1.018  volts,  then  7,840  +  100  = 
7,940.  Pointing  off  two  places,  gives  79.40,  which  multiplied  by  1.018 
gives  80.82  for  the  voltage  on  the  circuit. 

To  Measure  Internal  Resistance  of  Cell  with  Queen 
Acme  Set.— First  compare  its  voltage  on  open  circuit  with  the 
pressure  of  the  testing  set  battery..  Then,  shunt  the  cell  with  a 
known  resistance,  about  100  ohms,  and  again  measure  its  ter- 
minal voltage.     The  difference  between  the  two  values  thus 


Fig.  586. — Evershed  portable  ohmmeter  set.  This  testing  set  consists  of  a  direct  reading  ohm- 
meter  which  indicates  by  direct  reading  the  value  of  the  resistance  being  tested,  also  a 
portable  hand  dynamo  which  provides  at  any  required  pressure  the  current  necessary  to 
make  the  test.  It  is  adapted  to  the  needs  of  supply  stations,  wiring  contractors  and 
dynamo  builders.  It  is  also  useful  in  testing  the  insulation  of  underground  and  aerial 
cables,  and  is  designed  so  that  it  can  be  used  by  ordinary  workmen  who  are  not  experienced 
in  handling  delicate  instruments  and  who,  by  its  use,  are  able  to  obtain  accurate  results. 
The  dynamo  is  wound  for  100,  200,  500,  or  1,000  volts,  and  is  fitted  with  spring  drum 
inside  the  case  on  which  is  coiled  a  twin  flexible  cord  provided  with  a  connector  adapted 
for  clamping  under  the  ohmmeter  terminals. 

obtained,  divided  by  the  value  of  the  shunt  resistance,  will  give 
the  value  of  the  current.  To  find  the  internal  resistance,  mul- 
tiply the  value  of  the  shunt  resistance  by  the  ratio  between  the 
first  and  second  measured  values. 


510 


HAWKINS  ELECTRICITY 


For  instance,  assiune  that  the  open  circuit  voltage  of  the  cell  being 

tested  as  compared  with  the  voltage  of  the  testing  set  batten,'  is  .212 

of  the  latter,  and  that  when  it  is  shunted  with  a  resistance  of  1,000  ohms, 

its  terminal  voltage  is  .179.    Then,  the  total  resistance  is  to  the  1,000 

212 
ohms  shunt  resistance  as  .212  is  to  .179  or  ^^^^  X  1,000  =  1,184,  from 

.1<  9 

which  deducting  the  1,000  ohms  shimt  resistance,  gives  184  ohms  as  the 

internal  resistance  of  the  cell. 


Pic 


i.  lineman's  instniment  for  the  location  of  faults, 
-r.d  telegraph  circuits,  and  for  the  measurement 


Ammeter  Test  with  Queen  Acme  Set. — Connect  a  low 
resistance  in  series  with  the  ammeter  and  run  leads  from  it  to 

the  testing  set,  the  positive  lead  to  the  +  battery  post  and  the 
negative  lead  to  the  line  post  C  (fig.  577).    Insert  a  standard  cell 


TESTING  AND   TESTING  APPARATUS 


511 


between  the  battery  posts,  with  positive  terminal  to  +  battery 
post,  and  negative  terminal  to  —  battery  post.  Plug  com- 
mutator as  shown  in  fig.  582.  Remove  10,000  ohms  from  rheo- 
stat, and  100  ohms  from  bridge  arm  B.  Determine  a  balance  in 
the  usual  way  by  changing  the  value  of  the  resistance  in  the 


Fig.  588.— Diagram  showing  arrangement  and  connections  of  Leeds  and  Northrup  fault  finder 
It  IS  used  to  measure  conductor  resistance,  fault  resistance,  to  locate  faults  by  four  differ- 
ent tests  and  when  used  with  a  buzzer  and  telephone,  to  locate  opens.  The  essential 
teature  of  the  instrument  is  the  uniform  resistance  AB,  which  lies  in  a  circle  and  which 
has  a  value  of  about  100  ohms.  By  a  special  construction,  it  is  so  arranged  that  the 
contact  can  be  made  at  any  point  along  it,  and  it  is  therefore  equivalent  to  a  very  high 
resistance  slide  wire.  It  has  a  moving  contact  C  and  a  uniform  scale  of  1,000  divisions 
Xn  series  with  this  there  are  the  two  resistances  E  and  R  which  may  be  short  circuited 
by  the  switch^  U  and  V.  E  has  exactly  the  same  resistance  as  the  wire  AB.  R  has  a 
resistance  of  100  ohms,  and  is  the  fi.xed  resistance  of  the  bridge  arrangement  for  resistance 
measurements.  The  resistances  of  1.000  ohms  and  9,000  ohms  connected  to  the  battery 
post  are  to  protect  the  battery  and  the  apparatus  from  excessive  current.  The  9,000  ohms 
may  be  short  circuited  by  the  switch  W.  u""ia 


512 


HAWKINS  ELECTRICITY 


rheostat.  This  operation  will  balance  the  difference  of  pressure 
at  the  terminals  of  the  shunt  resistance  against  the  standard 
cell,  and  its  value  is  equal  to 


140 


1.40  X  100 


R  +  100         R  +  100 


To  determine  the  current  flowing,  divide  the  value  of  the 

difference  of  pressure  thus  obtained  by  the  value  of  the  shunt 

resistance. 

2o  VofH 


Swt  tch-Sethnq 

I  V. I  "^       \^       \J      '•~J\   '   I 

V     ^ 


Res. 


Fig.  5S9 — Resistance  measurement  ■nith  Leeds  and  Northrup  fault  finder.  The  diagram  shows 
the  proper  connections  and  switch  settings  for  measuring  conductor  resistance.  As  in 
the  ordinary  shde  wire  bridge,  the  resistance  X  between  the  two  posts  1  and  2  is 
obtained  from  the  formula  X  =  A  -i-  (1.000  -  A)  X  R.  To  avoid  the  necessity  of  sol\-ing 
in  each  case  the  fraction  A  -f-  (1.000-.\),  a  table  is  furnished  with  the  instrument,  giving 
the  value  of  this  fraction  for  each  value  of  A.  The  resistatue  is  accordingly  delermined  in 
each  case  by  simply  selling  the  contact  C  for  a  balance  and  reading  from  the  table  the  re- 
sistance opposite  the  number  corresponding  to  the  scale  reading  and  multiplying  by  loo,  the 
ralue  of  R.  To  use  an  outside  battery,  remove  the  inside  battery  and  connect  the  outside 
battery  t)etween  the  posts  Gr  and  Ba.  The  pressure  of  this  battery  should  not  exceed 
110    volts.    Jf  it  exceed  25  volts,  open  svfitch  IV. 

EXAMPLE — With  an  unknown  resistance  connected  between  the  posts  1  and  2.  the  galvano- 
meter showed  a  balance  for  a  dial  reading  of  387.  The  number  opposite  3S7  in  the  table 
is  .6313;  hence,  X  =  .6313  X  100  =  63.13  ohms. 


TESTING  AND   TESTING  APPARATUS 


513 


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o  5.5 


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L.  c.i2  «  u  «  o  J>  oote  o 

■o  E-g<c.S2  E  o..2^H-5  o 
u 


514 


HAWKINS  ELECTRICITY 


•O  O  J  •*  4)     -   ^  C 

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TESTING  AND   TESTING  APPARATUS 


515 


Loop  Test. — This  is  a  method  of  locating  a  fault  in  a  tele- 
graph or  telephone  circuit  when  there  is  a  good  wire  running 
parallel  with  the  defective  one.  In  the  process,  the  good  and 
bad  wires  are  joined  at  their  distant  ends  and  one  terminal-  of 
the  battery  is  connected  to  a  Wheatstone  bridge,  while  the 
other  terminal  is  grounded.  There  are  different  ways  of  making 
loop  tests  as  by : 


BATTERY  KEY 


Fig.  595. — The  Murray  loop  test.  The  apparatus  is  connected  as  in  the  figure.  The  rheostat 
of  the  bridge  is  used  in  place  of  the  second  arm  to  permit  large  adjustment.  X  and  Y  are 
the  resistances  of  the  cable  between  the  fault  and  the  points  1  and  2  respectively. 


1 .  The  Murray  loop ; 

2.  The  Varley  loop; 

3.  Special  loop. 


The  Murray  Loop. — In  this  test  only  one  of  the  two  regular 
bridge  arms  is  used,  the  other  being  replaced  by  the  rheostat 
giving  an  arm  of  large  adjustment. 


516 


HAWKINS  ELECTRICITY 


The  connections  are  shown  in  fig.  59o.  In  making  the  test,  close  key 
and  note  the  deflection  of  the  needle  due  to  pressure  of  chemical  action 
at  fault,  if  any.    This  is  called  the  false  zero. 

Now  apply  the  positive  or  negative  pole  of  the  battery  by  depressing 
the  battery  key,  and  balance  to  the  false  zero  previously  obtained  by 
var>'ing  the  resistance  in  arms  A  or  B.  Then  by  Wheatstone  bridge 
formula:  RX  =  AY,  and  L  =  X+Y;  Y  =  L  -  X,  whence 


X 


A 


R  +  A 


Y  = 


R 


B  +  A 


ooo-^ 


1000-^ 


^"'V 


Sw/tch-  Setfinq 

8  [m]  e-eQ 
[m)©— e  © 


Fig.  .596. — Murray  loop  method  of  fault  location  with  Leeds  and  Xorthrup  fault  finder. 
Case  I  where  there  are  two  wires  ha\'ing  equal  resistance,  in  one  of  which  there  is  a  fault. 
Connect  and  set  switches  as  shown;  join  the  good  wire  to  post  1  and  the  faulty  wire  to 
post  2.  The  resistance  of  E  is  equal  to  that  of  AB.  From  the  symmetry  of  the  arrange- 
ment, it  is  evident  that,  if  the  fault  were  exactly  at  the  junction  between  the  good  and 
bad  wires,  the  contact  point  C  would  rest  for  a  balance  at  1.000  on  the  scale,  or  at  500  if 
the  fault  were  half-way  along  the  bad  wire;  hence,  at  whatever  point  it  comes  to  rest,  the 
reading  divided  by  1,000  and  multiplied  by  the  length  of  the  bad  wire  is  the  distance 
from  the  instrument  to  the  fault. 

EXAMPLE — In  a  pair  of  equal  wires,  5.8  miles  long,  one  is  grounded.  With  the  connections 
made  as  above,  and  the  galvanometer  balanced  for  the  dial  reading  124,  the  distance  tc 
the  fault  is  (124  X  58)  ^  1,000  =  .7192  miles. 


Oues.     How  may  the  distance  from  2  to  the  fault  be 
determined  in  knots  or  miles. 

Ans.     Divide  Y  by  resistance  per  knot  or  mile. 


TESTING  AND   TESTING  APPARATUS 


517 


The  Varley  Loop. — This  is  a  method  of  locating  a  cross  or 
ground  in  a  telephone  or  telegraph  line  or  other  cable  by  using 
a  Wheatstone  bridge  in  a  loop  formed  of  a  good  wire  and  the 
faulty  wire  joined  at  their  distance  ends.  One  terminal  of  the 
battery  is  grounded  and  the  other  connected  to  a  point  on  the 
bridge  at  the  junction  of  the  ratio  arms.  The  rheostat  arm  then 
includes  the  resistance  of  the  rheostat  plus  the  resistance  of  the 


9000 


Swifch>-5eHir7(3. 

Se-e  e 


Fig.  597. — Murray  loop  method  of  fault  location  with  Leeds  and  Northrup  fault  finder: 
Case  II,  where  the  good  and  bad  wires  are  unequal.  The  figure  shows  the  connections. 
It  is  the  ordinary  Murray  loop  and  it  is  evident  that  the  resistance  a,  to  the  fault  will  be 
obtained  from  the  formula  a  =  (A  -^  1,000)  X  r,  where  r  is  the  resistance  of  the  loop, 
and  A  is  the  reading  of  the  contact  C  on  its  scale.  The  distance  d,  to  the  fault  is  obtained 
from  the  formula  d  =  Ar  ^  (1,000  X  M),  where  M  is  the  resistance  per  mile  of  the 
faulty  wire. 

EXAMPLE — A  wire  having  a  resistance  M  of  16.46  ohms  per  mUe  is  grounded.  This  wire  was 
looped  with  a  wire  of  unknown  resistance  and  the  total  resistance  of  the  loop  r  was  measured 
and  found  to  be  54.07  ohms.  Connections  were  made  as  in  the  figure,  and  the  reading 
A  was  found  to  be  332.  Substituting  these  values  in  the  above  formula:  d  =  (332  X  54.07) 
-T-  (1,000  X  16.46)  =  1.09  miles. 


fault,  while  the  unknown  arm  includes  the  resistance  of  the  good 
wire  plus  the  resistance  of  the  bad  wire  beyond  the  fault.  When 
the  bridge  is  balanced,  the  unknown  resistances  may  be  readily 
determined  by  a  simple  equation. 


518 


HAWKINS  ELECTRICITY 


In  making  the  Varley  loop  test,  the  resistance  of  looped  cable  or 
conductors  is  measured,  and  then  connected  as  in  fig.  598,  Close  the 
battery  key  and  adjust  R  for  balance. 

When  earth  current  is  present,  the  best  results  are  obtained  when  the 
fault  is  cleared  by  the  negative  pole,  and  just  before  it  begins  to  polarize. 
If  X  be  the  resistance  from  2  to  the  fault,  then 


X  = 


X        SffOUND 

-m r        ii 

B*TTEi?V  KV(      I ^  ||.' 


Fig.  598. — The  Varley  loop  test.  The  diagram  shows  the  various  connections.  X  and  Y  are 
the  resistances  of  the  cable  between  the  fault  and  the  points  1  and  2  respectively.  L  is 
the  resistance  of  the  good  and  bad  cable  or  X  +  Y. 


also,  X  divided  by  the  resistance  of  the  cable  or  conductor  per  knot  or 
mile  gives  the  distance  of  fault  in  knot  or  miles. 

When  the  resistance  of  the  good  wire  used  to  form  a  loop  with  the 
defective  wire,  together  with  that  portion  of  the  defective  ^vire  from 
the  joint  to  the  fault  is  less  than  the  resistance  of  the  defective  wire  from 
the  testing  station  to  the  fault,  the  resistance  R  must  be  inserted  be- 
tween point  1  and  the  good  conductor,  the  defective  wire  being  con- 
nected directly  to  point.    The  formula  in  this  case  is 


X  = 


L  +  R 


TESTING  AND   TESTING  APPARATUS 


519 


Special  Loop. — This  method  may  be  used  to  advantage  where 
the  length  of  the  cable  or  fatilty  wire  only  is  known  and  where 
there  are  two  other  wires  which  may  be  used  to  complete  the 
loop.  It  is  not  necessary  that  the  resistance  of  the  faulty  wire 
and  the  length  and  resistance  of  the  other  wires  be  known. 
Figs.  601  to  604  show  the  connections  and  method  of  testing. 

EXAMPLE.— All  the  wires  in  a  cable  10,852  ft.  long  were  found  to  be 
grounded  so  that  none  of  them  could  be  used  as  good  wires.  Two  wires 
were  selected  out  of  another  cable  going  to  the  same  place  by  a  different 


ffOOO 


Sv/itcf?- Setf  irx] 
V 


Figs.  599  and  600. — Varley  loop  method  of  fault  location  with  Leeds  and  Northrup  fault  finder. 
This  method  may  be  used  as  a  check  on  the  Murray  methods.  Connect  the  faulty  wire  to  1, 
and  measure  the  resistance  of  the  loop.  Then  throw  switches  as  shown  in  the  fig.  600. 
Let:  a  =  resistance  to  fault,  d  =  distance  to  the  fault  in  miles,  M  =  resistance  of  the 
faulty  wire  per  mile,  r  =  resistance  of  the  loop,  R  =  resistance  of  the  coil  R,  or  100 
ohms,  T  =  A  -7-  (1,000— A)  to  be  read  from  the  table.  From  the  Wheatstone  bridge 
relation:  a  =  (r-100  T)   -^  (T  +  D.  and  d  =  (r-100  T)   -J-  (T+1)  M. 

EXAMPLE — A  wire  having  a  resistance  of  16.46  ohms  per  mile  is  grounded.  This  was  looped 
with  a  wire  of  unknown  resistance  and  the  resistance  of  the  loop  was  found  to  be  54.07 
ohms.  Connections  were  made  as  in  the  figure,  and  the  reading  A  was  found  to  be  234. 
From  the  table  T  =  .3055,  and  substituting:  d  =  (54.07-30.55)  -r  (1.3055  X  16.46)  = 
1.094  +  miles. 


route  and  securely  joined  to  one  of  the  grounded  wires  at  the  distant, 
end.  This  grounded  wire  and  one  of  the  good  ones  were  connected  as 
shown  in  figs.  601  and  602  and  the  reading  A  was  found  to  be  307. 


520 


HAWKINS  ELECTRICITY 


Connections  were  then  made  as  shown  in  figs.  603  and  604  and  A  was 
found  to  be  610.    What  is  the  value  of  d? 
A  xx)rding  to  formula 

,       AL       307  X  10,853       ,  , .,  . 
^  =  "A    =  610 =  ^'^^^  ^'- 

The  Potentiometer. — For  the  rapid  and  acctirate  measure- 
ment of  voltage,  current,  and  resistance,  the  potentiometer  can 
be  recommended.     Those  in  charge  of  electric  light  and  power 


Svs/it'cb -Setting 

I  y. I V-/*    ^^    '^    '^l  '  I 


Pigs.  601  and  602. — Si)ecial  loop  test  with  Leeds  and  Northrup  fault  finder.  For  the  first 
measurement  connect  the  faulty  wire  to  2,  either  of  the  good  wires,  as  Z,  to  1,  the  post 
Gr  to  ground,  and  short  circuit  the  coils  R  and  E  by  closing  switches  U  and  V  as  in 
the  figures .  Balance  in  the  usual  way  and  call  the  dial  reading  A.  For  the  second  measure- 
ment, connect  the  post  Gr.  (disconnected  from  ground),  to  the  other  good  wire  y  as 
shown  in  figs.  603  and  604.  and  get  another  balance;  call  this  reading  A'.  The  distance 
d,  to  the  fault  is  determined  from  the  simple  formula  d  =  AL  -r  A'  where  L  is  the  length 
of  the  cable  or  faulty  wire. 


companies,    and   also   those   who   purchase  large  amounts  of 

electrical  energy  are  realizing,  more  and  more,  the  necessity  of 
having  satisfactory'  primary  standards  with  which  to  check  their 
volt-,  ampere-,  and  watt-meters. 


TESTING  AND   TESTING  APPARATUS 


521 


When  it  is  realized  that  an  error  of  one  per  cent,  in  a  com- 
mercial instrument  means  an  error  of  one  dollar  one  way  or 
the  other  in  every  one  hundred  dollars  charged,  the  need  of 
such  standardization  apparatus  becomes  at  once  apparent. 

The  potentiometer,  it  should  be  noted,  relies  for  its  accuracy, 
only  upon  the  constancy  and  accuracy  of  resistances  and  upon 
standard  cells. 


WWVSV- 


w 


Svifcb-Settipg. 
00^  © 


Figs.  603  and  604. — Special  loop  test  as  made  with  the  Leeds  and  Northrup  fault  finder. 
Diagram  showing  connections  for  the  second  measurement.  The  special  loop  test  may  be 
used  to  advantage  where  the  length  of  the  cable  or  faulty  wire  only  is  known,  and  where 
there  are  two  other  wires  which  may  be  used  to  complete  the  loop.  To  use  an  outside  bat- 
tery, connect  one  pole  to  Ba,  and  ground  the  other.  The  pressure  of  this  battery  must 
never  exceed  110  volts;  if  it  be  over  25  volts,  see  that  switch  W  is  open. 


With  the  materials  now  available,  and  the  skill  which  has  been 
acquired  in  their  manufacture,  both  the  resistances  and  the 
standard  cells  are  obtainable  which  are  remarkably  constant, 
and  both  can  be  readily  checked  for  accuracy. 


522 


HAWKINS  ELECTRICITY 


Location  of  Opens. — These  measurements  are  based  on  the 
fact  that  the  capacity  of  -^sires  in  a  cable  is  ordinarily  a  measur- 
able quantity,  which,  in  wire  of  uniform  diameter,  is  propor- 
tionate to  length.  In  making  these  tests,  a  fault  finder  is  used 
together  with  a  buzzer,  dry  cells  to  operate  it,  small  induction 
coil,  and  telephone  receiver.  These  instruments  are  to  be 
found  in  any  telephone  exchange.  It  is  best  to  locate  the 
buzzer  at  some  distance  from  the  fault  finder  in  order  that  it 
cannot  be  heard  by  the  operator. 


V 


Fig.  605. — To  use  galvanometer  of  Leeds  and  Northrup  fault  finder  in  series  with  the  battery: 
Set  switches  as  shown,  and  connect  between  posts  Gr.  and  2  (see  figs.  587  and  5&8).  The 
galvanometer  will  have  the  maTi'Tniim  sensibility  with  the  pointer  at  1,000  and  the  mini- 
mum at  zero. 


Fig.  606. — To  measure  high  resistances,  such  as  the  resistances  of  faults  with  Leeds  and 
Xorthrup  fault  finder.  First  Method. — Arrange  the  switches  as  shown  in  the  figure.  Con- 
nect p)Osts  Gr.  and  2,  turn  the  handle  until  the  galvanometer  needle  comes  to  rest  at  an  even 
deflection  of  ten  divisions.  Call  the  reading  A.  Connect  in  the  unknown  resistance 
between  Gr.  and  2.  Now  close  the  switch  W,  so  that  the  figure  1  appears  on  the  top  of 
the  block,  and  again  bring  the  galvanometer  to  a  deflection  of  ten  divisions  and  call  the 
reading  B.  Then  X  =  (10.000  B  -^  A)  -  1,000.  In  case  X  be  a  high  resistance,  it  will 
be  found  that  the  galvanometer  will  not  deflect  ten  di%-isions  for  any  position  of  the  pointer. 
In  such  case,  choose  a  number  of  di%-isions  which  is  a  factor  of  ten,  such  as  5.  2,  or  1,  and 
multiply  (10,000  B  -J-  A)  by  ten  di\'ided  by  the  number  chosen,  as  y  y .  y.  For  ex- 
ample, for  a  deflection  of  two  di^-isions:  X  =  ^  (10,000  B  -i-  A)  -  1,000.  The  satisfactory 
range  of  the  set  for  high  resistance  measurement  may  be  increased  by  using  an  outside 
batterv  of  higher  voltage.  ^Vilh  the  contained  batterj',  satisfactory  measurements  can 
be  made  up  to  1  or  2  megohms.  When  outside  battery  is  used,  connect  one  terminal  to 
the  j)Ost  ba,  and  the  other  to  2  for  the  reading  A.  Connect  the  batter>'  and  imknown 
resistance  in  series  between  these  posts  for  the  reading  B.  When  an  outside  pressure  of 
25  volts  or  over  is  ussd,  the  switch  W  should  not  be  closed  unless  there  be  a  resistance  in 
series  with  the  battery  of  10,CKX)  ohms  or  over.  Second  Method. — For  use  as  a  voltmeter 
to  measure  high  resistances.  (More  convenient  but  not  quite  as  accurate  as  first  method.) 
Set  the  switches  to  RV.  M  and  10.  Turn  the  knurled  nut  on  the  galvanometer  so  as  to 
set  the  needle  to  the  extreme  light  hand  side  of  the  scale.  Connect  the  posts  2  and  Gr. 
with  a  short  piece  of  wire.  Turn  the  rotating  pointer  on  the  scale  until  the  galvanometer 
needle  moves  over  about  20  scale  di%'isions  when  the  battery  key  is  closed.  Remove  the 
connection  between  2  and  Gr.  as  the  voltmeter  terminals.  This  makes  a  simple  way 
of  testing  for  various  kinds  and  amounts  of  trouble.  On  a  wet  cable  a  deflection  of  10 
to  15  di\Tsions  indicates  hea\->-  enough  trouble  to  locate  with  the  fault  finder.  With  a 
little  care,  trouble  showijis  only  5  or  6  divisions  can  be  located. 


TESTING  AND   TESTING  APPARATUS 


523 


Before  attempting  locations  for  opens  it  is  well  to  make  the 
following  measurements : 

1.  The  insulation  of  the  broken  wire  and  the  insulation  of  the 
good  wire  with  which  it  is  to  be  compared ; 

This  may  be  done  as  shown  in  fig.  606.  It  is  best  that  the  insulation 
resistance  be  fairly  good,  but  experiments  indicate  that  good  results  can 
be  obtained  by  the  methods  which  follow,  even  when  the  insulation  is 
as  low  as  100,000  ohms,  and  fair  results  when  as  low  as  50,000  to  100,000 
ohms. 


9O00 


Sw'ifchSetfing 


/V 


BAD    PA/R^ 


GOOD    f^/R 


Pig.  607. — Diagram  of  connections  in  testing  for  opens  with  Leeds  and  Northrup  fault  finder. 
The  apparatus  required  consists  of  fault  finder,  buzzer,  dry  cell  to  operate  buzzer,  small 
induction  coil,  and  telephone  receiver.  Connect  the  battery  to  the  primary  of  the  induc- 
tion coil,  one  terminal  of  the  secondary  to  the  post  Ba,  and  the  other  to  the  connected 
wires  as  shown.    Set  switches  U  and  V  so  as  to  short  circuit  the  two  resistance  coils. 


2.  The  resistance  between  the  two  sections  of  the  broken  wire 
:>hould  be  measured. 

This  may  be  done  by  joining  the  broken  wire  and  a  good  wire  at  the 
distant  end  of  the  cable  and  measuring  the  resistance  of  the  loop.  To 
ensure  close  locations,  this  resistance  should  be  over  100,000  ohms.    Fair 


524 


HAWKINS  ELECTRICITY 


locations  can  be  made  when  the  resistance  is  much  lower  and  it  is  worth 
while  to  attempt  it  even  if  the  resistance  be  as  low  as  10,000  ohms.  The 
difficulty  of  determining  the  balance  point  increases  as  the  resistance 
decreases. 

Oues.  Describe  the  method  of  locatmg  an  open  with  a 
fault  finder. 

Ans.  {Case  I)  The  broken  wire  \\'ill  be  one  of  a  pair.  Select 
another  pair  in  the  cable  that  will  have  the  same  capacity  per 
mile  and  join  together  the  mate  of  the  broken  wire  and  one  wire 


Pig.  608. — Diagram  of  connections  in  testing  with  Leeds  and  Northrup  fault  finder  for  open 
wire  in  telegraph  and  other  cables  in  which  the  wires  are  not  grouped  in  pairs.  Connect 
the  broken  wire  to  1.  Select  a  good  wire  and  join  to  2.  Connect  all  o:her  wires  and 
ground  them,  by  connecting  to  the  cable  sheath.  Connect  the  distant  end  of  the  broken 
wire  to  the  others.  Ground  the  end  of  the  induction  coil  that  is  not  connected  to  the 
post  Ba. 


of  the  other  pair.  Make  the  connections  as  shown  in  fig.  607, 
then  depress  the  battery  key  and  move  the  contact  to  the  point 
of  minimum  sound  in  the  telephone.  The  distance  to  the  break  is 
equal  to  LA  -^  (1,000 — A),  where  L  is  the  length  of  the  good  wire. 


TESTING  AND   TESTING  APPARATUS 


525 


EXAMPLE:  A  cable  1.45  miles  long  contained  a  broken  wire.  It  was 
found  that  the  insulation  resistance  of  the  end  of  this  wire  was  over  10 
megohms,  as  was  that  of  the  good  pair  selected  to  test  against  it.  The 
resistance  between  the  two  pieces  of  the  good  wire  was  also  over  10 
megohms.  Connections  were  made  as  in  fig.  607,  and  it  was  found  that 
the  balance  point  was  476.    Accordingly  from  the  table 


.9084 


and 


1,000  -  A 
d  =  1.45  X  .9084  =  1.317  +  miles. 


|«n|l|l| 1 

r-WS" 


•Switch -5ctf  I  pg. 


BAD    R^/R 


N 


GOOD  PAIR 


Fig.  609. — Diagram  of  connections  for  reading  Tn  testing  for  opens  with  Leeds  and  Northrup 
fault  finder,  when  broken  wire  and  good  wire  are  not  in  the  same  cable. 


Location  of  Opens. — {Case  II)  Open  wire  in  telegraph  or 
other  cables  in  which  the  wires  are  not  grouped  in  pairs.  The 
connections  are  made  as  in  fig.  608,  and  the  measurement  and 
calculation  exactly  as  in  the  preceding  case. 

The  accuracy  of  the  location  of  both  of  the  above  methods  depends  on 
the  good  and  broken  pair,  or  the  good  and  broken  wires  having  equai 
and  uniform  capacity  per  unit  length.    It  is  not  always  possible  to  select 


526 


HAWKINS  ELECTRICITY 


wires  that  are  alike  in  this  respect.  In  such  cases,  as  for  instance,  where 
there  is  no  good  wire  in  the  cable  containing  the  broken  wire,  and  a  good 
wire  has  to  be  selected  from  another  cable,  the  method  of  C4ise  III  may 
be  used. 

Location  of  Opens. — Case  III,  in  which  the  broken  wire  and 
good  Vi-ire  are  nor  in  the  same  cable.  Connect  the  good  wire 
and  broken  wire  in  the  same  way  as  shown  in  fig.  607,  and  set 


Pic.  610. — ^Leeds  and  Northnip  potentiameter.  It  is  direct  reading  from  .000001  vott  to  16 
\'olts,  asd  with  accessories  tiie  range  may  be  extended  to  1600  volts,  and  cmrents  may 
be  measured  up  to  3CNX)  amperes.  The  instrument  has  fifteen  cofls  of  5  ohms  eac^,  wfaicfa 
are  in  series  vith  an  extended  trire  aboat  190"  long  of  equal  leastance.  Tbe  dectrical 
circuits  are  shown  in  tbe  diagram  fig.  611.  It  is  well  for  the  user  to  open  np  the  potenti> 
ometer  and  make  himself  familiar  with  its  interior  constructi<»,  in  otxler  to  fnlly  under- 
stand the  operation  of  the  rheostat  and  other  pans.  There  are  no  con  tart  resastanoes  in 
the  potentiometer  drcnit  proper.  The  potentiameter  has  low  internal  resistance  wfaidi 
gives  it  the  maximum  sensibility.  Compared  with  faigfa  resistance  potentiameter,  tins  is 
especially  advant.ageous  in  measuring  the  e!ectroax>tive  force  at  tbennooonples.  and  the 
fall  of  potential  acrc'ss  s:.andard  low  resistances.  As  ooostmctad.  tbe  last  ooe^tenth  volt  is 
covered  by  the  extended  wire  and  the  handle  which  carries  the  oootact  potat  on  tbe  wire 
may  be  manipulated  rapidly  so  that  a  fluctuating  voltage  may  be  aoctnatdy  followed. 
When  used  with  any  cadmium  cell,  the  pipteniiometer  is  direct  reading.  The  accuracy  of 
the  potentiometer  resistances  can  be  voified  with  the  facilities  of  the  ofdinuy  labamtory. 


the  pointer  for  a  balance.      Call  the  reading  A.     Then  connect 
the  good  wire  and  the  broken  wire  at  the  distant  end  and  set  the 


TESTING  AND   TESTING  APPARATUS 


527 


-Ba. 


Fig.  611. — Diagram  showing  connections  of  Leeds  and  Northrup  potentiometer.  The  coils  in 
the  series  AD  are  each  o  ohms,  and  between  each  two  there  is  a  brass  block  with  a  reamed 
hole.  A  pair  of  flexible  cords  with  taper  plug  terminals  to  fit  these  holes  is  furnished. 
These  coils  can  be  measured  with  an  ordinary  Wheatstone  bridge  and  thus  compared 
with  each  other  to  a  high  degree  of  accuracy,  even  if  the  bridge  be  not  accurate.  For 
potentiometer  work,  the  essential  point  is  that  they  should  be  like  each  other,  not  that 
they  should  be  accurately  any  particular  value.  In  the  same  way  the  resistance  of  the 
extended  wire  can  be  compared  with  the  resistance  of  the  coils  in  AD.  Its  resistance 
should  be  1.1  times  the  value  of  any  coil  between  A  and  D.  Outside  connection  with  the 
extended  wire  may  be  made  by  using  the  posts  marked  BR  and  — BA.  This  adjustment 
for  balancing  an  unknown  electromotive  force  is  accomplished  by  the  manipulation  of  the 
two  contact  points  M  and  M'.  The  coils  AD  are  arranged  in  a  circle,  a  revolving  switch 
moving  M.  A  checking  device  enables  the  operator  to  set  this  switch  without  taking  his 
eye  from  the  galvanometer.  The  resistance  S  is  of  such  value  that  when  it  shunts  the 
wire  OB,  the  total  resistance  between  O  and  B  is  Jq  of  the  same  urrshunted.  When  the 
shunt  is  applied,  provided  the  total  current  remain  the  same,  the  drop  between  any  two 
points  on  AB  will  be  ^,  of  its  previous  value.    The  total  current  will  remain  the  same 

provided  the  total  resistance  in  the  circuit  remain  the  same.  This  is  accomplished  by 
making  the  coil  K  such  that  it  exactly  compensates  for  the  reduction  in  resistance  caused 
by  plugging  in  the  shunt  coil  S.  The  low  scale  is  applied  by  moving  the  plug  from  the 
position  1  to  the  position  .1.  With  this  change  the  potentiometer  reads  from  .16  volt 
down  by  indicated  steps  of  .000005  volt.  The  reading  is  very  simple.  For  instance,  if  M 
stand  at  1.2  and  M'  at  1.35  revolutions,  the  reading  is  1.2135  volts.  The  resistances  of 
the  instrument  are  wound  upon  metal  spools,  and  are  therefore  able  to  dis.sipate  a  com- 
paratively large  amount  of  energy.  This  allows  the  potentiometer  to  be  used  for  pressure 
measurements  up  to  16  volts  without  the  use  of  a,  volt  box. 


528 


HAWKINS  ELECTRICITY 


pointer  for  a  new  balance.      Call  this  A'.      The  connections  for 
this  reading  are  sho\^Ti  in  fig.  609.     The   distance  to  the  break 

will  be 

A  A'L 

1,000  (A  -  A' )  +  A  A' 

where  L  is  the  total  length  of  the  broken  wire. 


^jTb  Wtffry 


A  5^ 


Pig.  612. — Diagraxa  showing  actual  connections  in  the  rheostat  of  Leeds  and  Xorthrup  potentio- 
meter. The  figure  corresponds  to  R  of  fig.  611.  The  rheostat  is  mounted  in  the  end  of 
the  potentiometer  as  shown  in  fig.  610.  Rough  adjustment  of  the  potentiometer  ctirrent 
is  made  by  means  of  the  variable  resistance  R.  Fine  adjustment  is  made  by  means  of  the 
variable  resistance  R'.  It  will  be  noted  that  the  23  ohm  resistance  of  this  latter  rheostat 
is  shunted  by  a  resistance  of  6.1  ohms,  making  possible  a  very  fine  regulation.  Further, 
there  is  in  series  with  the  moving  contact  a  resistance  of  400  ohms,  which  makes  the  effect 
of  variable  contact  resistance  negligible.  Only  one  cell  of  storage  battery  should  be  used. 
When  this  battery  is  fresh,  the  plug  shown  in  the  figure  at  2R  should  be  inserted  at  R. 
This  gives  the  greatest  resistance  in  the  rheostat  circuit.  As  the  cell  runs  down,  the  plug 
should  be  changed  to  2R.  When  both  plugs  are  in,  the  rheostat  slide  wires  are  in  series 
with  the  potentiometer  circuit. 


EXAMPLE :  A  pair  of  wires  containing  one  broken  wire  was  connected 
with  a  good  pair  in  a  different  cable  as  shown  in  fig.  607.  The  reading 
A  was  found  to  be  180.  The  good  and  bad  wires  were  then  joined  at  the 
distant  end  as  in  fig.  609,  and  the  reading  A  was  found  to  be  88.  The 
total  length  of  the  bad  wire  MX  was  1.44  miles.  Required,  the  distance 
to  the  break.    Substituting  the  values  in  the  formula: 


d  = 


180  X  88  X  1.44 


1,000  (180  -  SS>  +  ISO  X  88 


=  .211  +  mile. 


TESTING  AND   TESTING  APPARATUS 


529 


To  Pick  Out  Faulty  Wires  in  a  Cable. — Short  circuit  the 
coils  E  and  R  with  switches  U  and  V,  Set  the  pointer  at  1,000. 
Connect  the  post  Gr.  to  ground  or  the  cable  sheath  and  apply 
the  wires  one  after  another  to  the  binding  post  2.  The  gal- 
vanometer will  deflect  for  a  faulty  wire. 


Fig.  613. — Diagram  of  the  Crompton  potentiometer.  In  this  instrument  the  resistance  con- 
sists of  fourteen  coils,  each  of  10  ohms,  in  series  with  a  straight  wire,  also  10  ohms  resistance, 
thus  forming  a  system  of  fifteen  equal  steps.  Across  the  whole  a  pressure  of  1.5  volt  is 
applied  from  a  secondary  cell,  thus  providing  .10  volt  per  step.  Any  fraction  is  then 
tapped  off  by  means  of  a  radial  switch  on  the  resistance  coils  and  a  sliding  contact  on  the 
wire.  The  standardization  is  performed  by  adjusting  a  resistance  in  series  with  the  whole 
until  the  standard  cell  employed  indicates,  by  means  of  the  galvanometer  G,  a  balance  at 
the  point  which  represents  its  electromotive  force  on  the  basis  given  above. 

Oues.    What  is  a  potentiometer? 

Ans.  An  arrangement  of  carefully  standardized  resistances 
for  measuring  voltages  in  comparison  with  a  standard  cell.  It 
is  used  for  accurate  measurement  of  voltages,  currents,  and 
resistances. 

In  place  of  a  series  of  standardized  resistances,  a  slide  wire  may  be 
used  as  in  fig.  614. 


Ones.    Describe  one  form  of  potentiometer. 

Ans.     As  shown  in  fig.  614,  it  consists  of  a  fine  German  silver 
wire  about  3  feet  long  stretched  between  the  binding  posts  A,  B, 


530 


HA  WKIXS  ELECTRIC  I T  Y 


which  are  attached  tx)  a  wooden  base  carrying  a  scale  diNnded 
into  1,000  equal  parts.  There  are  three  circuits,  the  terminal  A 
being  included  in  each,  one  including  the  batter}%  and  the  other 
two  the  galvanometer.  A  three  point  switch  connects  the 
galvanometer  in  series  with  the  standard  cell  SC,  or  the  cell  to 
be  tested  C,  the  circuits  being  completed  by  leads  terminating 
in  the  sliding  contacts  M  and  S. 


^^SmiCH 


OJUSTABLE 
ESISTi^NCE 


Fig.  614. — Digram  <rf  potentiometer  showing  method  of  measuring  the  voltage  of  a  cett. 
The  potentaooieter  is  simply  a  high  resistance  wire  of  uniform  diameter  stretched  between 
two  biadillt,  posts,  A  and  B,  in  such  a  way  that  contact  can  be  made  at  its  ends  and  along 
its  Ift^h-  Xecessan'  circuits  are  plainly  shown  in  the  figure;  SC,  is  a  standard  cttl  and 
C.  the  odl  to  be  tested.     M,  and  S  are  ^ding  contacts,  connecting  with  the  "slide  wire." 

Oues.  Describe  the  method  of  measuring  the  voltage 
of  a  cell  with  a  potentiometer. 

Ans.  Fig.  614  shows  a  method  of  comparing  a  pressure  with 
that  of  a  standard  cell  and  is  applicable  whether  the  pressure  of 
the  cell  to  be  tested  be  greater  or  less  than  that  of  the  standard 
cell.  In  making  the  test  the  switch  F  is  first  closed,  then  the 
other  s'witch  is  moved  to  D,  and  M  adjusted  till  galvanometer 
shows  no  deflection;  similarl3^  the  switch  is  moved  to  G,  and  S 
adjusted  till  galvanometer  shows  no  deflection.  Then, 
C  :  SC  =  AS  :  AM.  from  which  C  =  SC  X  AS  -r-  AM. 


TESTING  AND   TESTING  APPARATUS  531 


EXAMPLE. — Let  1.016  volts  be  the  known  voltage  of  the  standard  cell 

SC,  and  the  scale  reading  of  AS  be  657,  and  of  AM,  225  as  in  the 

figure,  then 

„      1.016  X  657     __-_      ,^ 
C  = ;prr- =  2.966  volts 

The  arrangement  may,  however,  be  made  direct  reading,  that  is,  the 
slide  wire  may  have  a  scale  of  volts  instead  of  lengths  or  resistances,  as 
follows:  Suppose  the  standard  cell  to  have  a  pressure  of  1.434  volts,  the 
sliding  contact  M  is  placed  at  the  reading  1.434,  and  the  adjustable 
resistance  varied  till  the  galvanometer  shows  no  current.  This  means 
that  the  pressure  between  A  and  M  is  1.434,  and  consequently  the  pres- 
sures all  along  the  slide  can  be  read  oflf  the  scale  in  volts.  Hence,  when  S 
has  been  adjusted  to  balance,  the  pressure  of  Cis  read  off  the  scale  in  volts. 

How  to  Use  a  Potentiometer.. — All  connections  must  be 
made  as  indicated  by  the  stamping  on  the  instrument.  Particular 
attention  must  be  given  to  the  polarity  of  the  standard  cell,  of 
the  battery,  and  of  the  voltage,  the  corresponding  +  and  —  signs 
being  marked.  If  used  with  a  wall  galvanometer  having  a 
telescope  and  scale,  it  will  be  found  convenient  to  place  the 
potentiometer  so  that  the  telescope  is  directly  over  the  glass 
index  of  the  extended  wire,  thus  permitting  the  observer  to  read 
the  galvanometer  deflections  and  potentiometer  settings  with- 
out changing  his  position. 

Potentiometer  Current. — A  medium  sized  storage  cell  will  be 
found  desirable,  producing  a  steady  current.  Errors  in  measurements 
are  frequently  made  by  using  an  unsteady  current. 

Setting  for  Standard  Cell. — Set  the  standard  cell  to  correspond 
with  the  certified  pressure  of  the  standard  cell  as  given  in  its  certificate. 
In  using  the  potentiometer  shown  in  fig.  610,  place  the  plug  in  hole  1, 
and  see  that  it  is  always  in  this  position  when  checking  against  the 
standard  cell.    Place  the  double  throw  switch  at  STD.  CELL. 

Adjust  the  regulating  rheostat  until  the  galvanometer  shows  no 
deflection.  In  making  the  first  adjustment  use  the  key  marked  Ri; 
when  a  balance  is  almost  attained,  use  key  R2,  and  for  the  final 
adjustment  use  key  marked  Ro.  This  cuts  out  the  resistance  in  series 
with  the  galvanometer  and  gives  the  maximum  sensibility. 

Measurement  of  Unknown  Pressure. — The  potentiometer  (fig. 
610),  as  ordinarily  used,  gives  direct  readings  for  voltages  up  to  and 
including  1.6  volts.  For  pressures  higher  than  1.6  volts,  a  volt  box  or 
multiplier  should  be  used.    After  obtaining  the  standard  cell  balance, 


532 


HAWKINS  ELECTRICITY 


as  previously  described,  place  the  double  throw  switch  in  the  position 
marked  E.M.F.  The  balance  for  the  unknown  E.M.F.  is  obtained  by 
manipulating  the  tenths  switch  and  rotating  the  contact  on  the  extended 
potentiometer  wire.  The  final  position  of  the  two  contacts  in  conjunction 
with  the  position  of  the  plug  at  the  left  of  the  instrument  indicates  the 
voltage  under  test. 

As  directed  above,  use  key  Ri  for  rough  adjustment,  R2  for  intermediate 
adjustment,  and  key  Ro  for  final  adjustment. 

Plug  at  1  gives  readings  for  voltage  directly  from  settings  of  tenths 
switch  and  extended  wire  contact. 

Plug  at  .1  shunts  the  potentiometer  circuit  so  that  the  voltage  meas- 
ured is  ,1  of  the  reading  taken  directly  from  the  scale.  Hence,  the 
readings  taken  from  the  setting  of  the  tenths  switch  and  the  slide  wire 
contact  must  be  divided  by  10. 


EMi: 


Fig.  615. — To  measure  a  pressure  greater  than  1 .6  volts  with  Leeds  and  Nothrup  potentiometer 

by  using  a  volt  box  or  multiplier.  To  measure  high  voltages  it  is  necessary  to  connect  the 
voltage  across  high  resistance  and  to  measure  on  the  potentiometer  a  definite  fraction  of 
the  total  drop.  In  the  figure,  AB  is  a  high  resistance  of  which  CB  is  .1th,  DB  .01th,  and 
EB  .001th  of  the  total  resistance.  The  potentiometer  reading  is  accordingly  multiplied 
by  10,  100,  or  1,000,  depending  upon  whether  the  switch  M  is  set  on  C,  D.or  E.  Resistance 
boxes  lor  this  purpose  are  called  volt  boxes,  and  are  constructed  to  multiply  the  potentio- 
meter readings  by  10,  100,  and  1,000.  In  using  them,  it  is  only  necessary  to  connect  the 
unknown  E.M.F.  at  the  posts  so  marked,  and  the  potentiometer  to  the  posts  marked  P. 
The  potentiometer  reading  is  taken  as  above  and  multiplied  by  a  factor  depending  upxsn 
the  position  of  the  switch  M,  which  factor  is  indicated  upon  the  box.  It  is  essential  in 
making  these  connections  that  the  polarity  be  carefully  observed. 


To  Balance  Galvanometer  for  Unknown  Voltage. — Place  plug 
in  hole  1  (fig.  610)  for  voltages  up  to  1.6,  and  in  hole  .1  for  voltages  up 
to  .16.  Rotate  the  tenths  switch  until  a  condition  of  balance  is  obtained 
exactly  or  approximately.  To  secure  an  exact  balance,  rotate  the  con- 
tact on  the  extended  wire.  The  unknown  voltage  can  now  be  read 
directly  from  the  position  of  the  tenths  switch  and  the  extended  wire 
Qontact  if  plug  be  at  1,  or  by  dividing  by  10  if  plug  be  at  .1. 


TESTING  AND   TESTING  APPARATUS 


533 


EXAMPLE. — A  balance  was  obtained  with  the  tenths  switch  at  1.3,  the  extended  wire  con- 
tact at  176  and  the  plug  at  1.  The  voltage  under  test,  therefore,  is  1.3176.  If  the  plug  at  .1 
had  been  used,  the  same  reading  would  have  indicated  .13176. 


To  ascertain  if  the  current  in  the  potentiometer  circuit  has  altered 
during  a  measurement,  it  is  only  necessary  to  plug  in  at  1,  place  the  double 
throw  switch  on  STD.  CELL  and  close  the  galvanometer  key.  No 
deflection  indicates  that  the  current  has  not  changed.  If  the  galva- 
nometer deflect,  the  regulating  rheostat  must  again  be  adjusted  until 
the  galvanometer  shows  no  deflection. 


F:g  616. — Measurement  of  current  with  potentiometer.  This  is  done  by  measuring  the  drop 
in  volts  across  a  known  low  resistance.  In  the  figure  S  is  the  standard  resistance,  and  on 
it  are  the  pressure  terminals  pp,  and  the  current  terminals  CC.  The  potentiometer  is 
connected  to  the  shunt  through  the  posts  marked  P.  The  resistance  between  the  points  pp 
is  adjusted  to  an  even  fraction  of  an  ohm.  These  resistances  are  so  chosen  that  in  order 
to  determine  the  current  passing  through  the  shunt,  after  having  obtained  a  potentiometer 
balance  it  is  only  necessary  to  multiply  the  potentiometer  reading  by  a  simple  factor.  For 
instance,  in  using  a  .01  ohm  standard,  it  is  only  necessary  to  multiply  the  potentiometer 
reading  by  100,  which  gives  the  current  reading  in  amperes;  similarly,  a  .1  ohm  requires 
multiplication  by  10,  and  a  .001  ohm  by  1,000. 


To  Measure  Voltages  from  1.6  to  16.— Pressures  up  to  16  volts 
may  be  measured  by  using  a  greater  voltage  across  the  BA  posts  (fig. 
615).  For  this  purpose  a  battery  of  about  20  volts  should  be  used. 
Insert  the  large  plug  at  .1  and  throw  the  switch  to  STD.  CELL,  then 
balance  the  galvanometer  by  means  of  the  regulating  rheostat.  When 
the  rheostat  has  been  set  to  secure  a  balance,  insert  the  large  plug  at  1, 
set  the  switch  on  E.M.F.  and  read  the  voltage  in  the  usual  manner! 
Multiply  the  reading  by  10. 


Care  of  Potentiometer. — The  slide  wire,  although  pro- 
tected to  a  great  extent  by  the  hood,  in  time  accumulates  dust 
and  dirt  with  a  thin  film  of  oxide.     This  will  tend  to  increase 


534 


HAWKINS  ELECTRICITY 


the    resistance   in    this    part    of    the    circuit    o^ving    to    poor 
contact.     This  -wire  should,  therefore,  be  cleaned  occasionally. 

To  do  this,  unscrew  the  stop  against  which  the  hood  strikes  when 
turned  to  read  zero;  then  remove  the  hood  and  rub  the  entire  sUde  wire 
vigorously  with  a  soft  cloth  dipped  in  vaseline.  Do  not  use  emery  or  sand 
paper  as  this  uill  destroy  the  uniformity  of  the  slide  -wire.  Clean  also  the 
steel  contact  which  rubs  on  the  wire,  as  this  becomes  glazed  after  much 
use.  When  the  potentiometer  is  not  in  use,  the  hood  should  be  screwed 
all  the  way  down,  and  the  lid  put  in  place  to  exclude  dust. 

If  it  be  used  in  a  chemical  factor}',  laboratory,  or  any  place  where  acid 
fumes  are  prevalent,  this  latter  precaution  i?  important,  because  the 
fumes  may  attack  the  slide  wire. 

It  is  also  well  to  keep  the  contact  surfaces  of  the  switch  studs  clean  and 
bright  by  wiping  them  occasionally  \snth  a  soft  cloth  dipoed  in  vaseline. 


yftf.   e/  leads  on  each  end  li  eoutJ  fo  10  dinuoni 
ef  slie/e  ivire.''  The  j/tde  w/ire  n  dvided  mfa 
1000  paiii  -20  for  the  Uadi,  or  S80  ditijiam  . 
Calibrated  Scale  en    GaUanomefmr 


Ir 


•^s 


Fig.  617. — Diagram  of  Leeds  and  Korthrup  bridge  for  locating  faults  in  power  circuits,  showing 
arrangement  of  the  connections  including  the  lead  cables  and  galvanometer  contacts. 
Make  connections  as  shown.  The  clamps  must  be  so  fastened  at  K  and  C  that  the  contact 
resistances  will  be  very  small.  This  contact  resistance  will  figure  as  an  error  in  the 
measurement.  If,  for  instance,  the  contact  resistance  were  equal  to  .001  of  an  ohm,  and 
the  wire  were  of  such  a  size  that  .001  of  an  ohm  were  equal  to  the  resistance  of  20  feet  of 
the  cable,  there  would  be  an  error  of  20  feet  in  the  location  of  the  fault.  For  this  reason 
all  contact  resistances  throughout  the  loop  from  \  to  C  must  be  extremely  small.  The 
battery  is  to  be  connected  to  the  posts  marked  Ba..  and  the  post  marked  Gr.  is  to  be 
grounded.  It  will  very  frequently  happen  that  the  ground  is  to  the  cable  sheath  or  some 
other  conductor.  In  this  case,  the  binding  post  Gr.  should  be  grounded  to  this  conductor. 
Sufficient  battery  should  be  used  to  give  a  readable  deflection  on  the  galvanometer  for  a 
small  movement  of  the  contact  on  the  bridge  wire.  1  he  fault  is  located  by  the  usual 
Murray  formula.  If,  for  instance,  the  galvanometer  show  no  deflection  when  the  contact 
is  at  300  on  the  scale,  it  would  indicate  that  the  fault  is  at  a  distance  from  A  equal  to  .003 
of  the  total  length  of  the  loop  from  A  to  C.  _  A  testing  current  of  five  amperes  may  be 
used  ^\4th  this  bridge.  Incasesof  necessity,  this  c'urrent  may  beincrcasid  to  eight  amperes, 
but  when  this  current  is  used  it  should  not  be  allowed  to  pass  through  the  bridge  for  a 
longer  time  than  i  s  necessary.  It  frequently  happens  that  small  faults  which  have  a  very 
high  resistance  develop  in  high  pressure  cables.  Such  faults  are  likely  to  break  down  and 
result  in  damage  and  should  be  located.  It  is  usually  impossible  to  locate  these  faults 
until  they  have  been  partially  carbonized.  This  must  be  done  by  apph-ing  a  sufficiently 
high  voltage  between  the  cable  and  the  sheath  (or  whatever  it  is  grounded  on)  to  break 
down  the  fault.  In  order  to  prevent  the  breaking  down  process  from  resulting  in  a  serious 
bum  out,  a  high  resistance  must  be  placed  in  the  circuit  which  will  prevent  an  excessive 
current,  or  the  circuit  n:ust  be  carefully  fused.     The  former  procedure  is  the  better. 


TESTING  AND    TESTING  APPARATUS  535 

Location  of  Faults  where  the  Loop  is  Composed  of 
Cables  of  Different  Cross  Sections. — Faults  in  loops  of  this 
character  may  be  located  with  the  same  degree  of  accuracy  as 
those  in  loops  of  a  uniform  cross  section,  provided  the  length 
and  cross  section  of  each  length  of  cable  are  known.  An  example 
will  illustrate  the  method: 


Fig.  618. — The  Fischer  portable  cable  testing  set,  designed  for  locating  crosses,  grounds  and 
breaks  in  cables,  also  for  conductor  and  liquid  resistance  measure.  The  distinguishing 
feature  of  the  set  is  the  master  switch.  By  means  of  this  switch,  connections  can  he  made 
for  the  various  tests  by  a  single  movement,  thus  avoiding  the  labor  and  time  which 
have  to  be  expended  in  interchanging  the  connections  and  memorizing  the  rather 
complicated  scheme  of  connections. 

In  the  diagram,  fig.  617,  assume  the  length  of  the  cable  AE 
to  be  550  yards  of  25,000  cir.  mil.,  EF,  500  yards  of  40,000  cir. 
mil.,  and  FC,  1,050  yards  of  30,000  cir.  mil.  These  lengths 
must  be  reduced  by  calculation  to  equivalent  lengths  of  one 


536  HAWKINS  ELECTRICITY 

size,  and  for  this  purpose  it  is  best  to  select  the  largest  size. 
The  results  of  this  calculation  are  as  follow's: 

550  yds.  of  25,000  dr.  mil.    =     880  yds.  of  40,000 cir.mil. 
500    "     "    40,000  "       "      =     500     "     "  40,000  "     " 
1,050     "     "    30,000  "       "      =1,400     "     "  40,000  "     " 

This  makes  the  total  resistance  of  the  loop  equivalent  to 
2,780  yards  of  40,000  dr.  mil.  If  the  contact  show  a  balance 
for  a  reading  of  372.5,  this  indicates  that  the  fault  is  at  a  dis- 
tance of  tM  of  2,780  =  1,035.5  equivalent  yards.  Of  this,  880 
are  in  the  stretch  A  E.    Consequently  the  fault  is: 

1,035.5  -  880  =  155.5  yards  from  E. 


AMMETERS.    VOLTMETERS,  AND    WATTMETERS         537 


CHAPTER  XXVIII 

AMMETERS,   VOLTMETERS   AND 
WATTMETERS. 


An  ammeter  or  ampere  meter  is  simply  a  commercial  form  of 
galvanometer  so  constructed  that  the  deflection  of  the  needle 
indicates  directly  the  strength  of  current  in  amperes.  A  good 
ammeter  should  have  a  very  low  resistance  so  that  very  little  of 
the  energy  of  the  current  will  be  absorbed ;  the  needle  should  be 
dead  beat,  and  sufficiently  sensitive  to  respond  to  minute  varia- 
tions of  current. 

According  to  the  principle  of  operation,  ammeters  and  volt- 
meters are  classified  as: 

1.  Moving  iron; 

2.  Moving  coil; 

3.  Solenoid  or  plunger; 

4.  Magnetic  vane ; 

5.  Hot  wire; 

6.  Electrostatic; 

7.  Astatic; 

8.  Inclined  coil; 

9.  Fixed  and  movable  coil. 

Again,  they  are  divided  according  to  their  use  into  two  classes: 

1.  Portable  type; 

2.  Switchboard  type; 


538 


HAWKINS  ELECTRICITY 


MiUi-ammeters  or  milli-voltmeters  are  instruments  in  which  the  scale 
is  graduated  to  read  directly  in  thousandths  of  an  ampere  or  thousandths 
of  a  volt  respectively. 

Ques.     Describe  the  moving  iron  type  instrument. 

Ans.  The  arrangement  of  the  working  parts  are  shown  in 
jg.  620.  A  soft  iron  needle  N,  is  pivoted  inside  of  a  coil  C,  and 
is  held  out  of  line  with  the  axis  of  the  coil  by  means  of  a  permanent 
magnet  M,  when  the  instrument  is  idle.  In  this  position,  a 
pointer  P,  which  is  attached  to  the  needle,  stands  at  the  zero 
mark  of  the  scale  S.      If  a  current  be  passed  through  the  coil, 


Fig.  620.  MoWng  iron  type  instrument.  The  essential  parts  are:  N,  soft  iron  needle;  C, 
coil;  M,  permanent  magnet;  P.  pointer;  S.  scale.  Current  passing  through  the  coil  acts 
on  the  needle,  causing  it  to  turn  against  the  restraining  force  due  to  the  influence  of  the 
permanent  magnet. 

magnetic  lines  of  force  are  set  up  in  its  center,  which  tend  to  pull 
the  needle  into  line  -^-ith  them,  and  therefore  \\-ith  the  axis  of  the 
coil.  This  pull  is  resisted  by  the  permanent  magnet  M,  and  the 
amount  of  deflection  of  the  needle  from  the  zero  position  depends 
upon  the  strength  of  the  current  or  the  voltage  according  as  the 
coil  is  wound  to  indicate  amperes  or  volts. 


AMMETERS.    VOLTMETERS,  AND   WATTMETERS        539 


Ques.    Describe  a  moving  coil  instrument. 

Ans.  This  type  of  instrument  is  shown  in  fig.  621.  It  consists 
of  a  moving  coil  C,  to  which  is  attached  the  pointer,  and  which  is 
pivoted  between  the  poles  of  a  permanent  magnet  M.  The  coil 
moves  between  these  poles  and  a  fixed  soft  iron  core  K,  and  is 


Fig.  621. —  Moving  coil  type  instrument.  The  essential  parts  are:  A,  spiral  spring;  C  coil; 
K,  soft  iron  core;  M,  permanent  magnet;  P,  pointer;  S,  scale.  Current  passing  through 
the  coil  causes  the  moving  system  to  turn  against  the  restraining  force  due  to  the  influ- 
ence of  the  permanent  magnet. 

held  in  the  normal  position  by  two  spiral  springs  A,  above  and 
below  the  core.  The  springs  also  serve  to  make  electrical  con- 
nection with  the  coil  C. 

When  a  current  passes  through  the  coil,  magnetic  lines  are  set  up  in 
it  which  are  at  an  angle  to  those  passing  from  one  pole  of  the  permanent 
magnet  to  the  other.  The  lines  of  force,  which  formerly  passed  from 
one  pole  of  the  magnet  to  the  other  by  straight  lines  or  by  short  curved 
ones,  are  "  stretched"  on  account  of  the  field  produced  by  the  current 
in  the  coil,  and,  in  trying  to  shorten  themselves,  tend  to  twist  the  coil 
through  an  angle.  This  tendency  to  move  is  resisted  by  the  two  spiral 
springs,  hence  the  coil  moves  until  equilibrium  is  established  between 
the  two  opposing  forces. 

The  amount  of  deflection  of  the  pointer  depends,  either 
upon  the  current  strength,  or  the  voltage  according  to  the 
winding  of  the  coil. 


540 


HAWKINS  ELECTRICITY 


c  —      u  2  Be  ^  c 
Mr::    .«  o  c  >.— 

C  i)      ZX  —7"  o  -  =  =3 


c  '  -  =  o  ='-S'=3 

'^  5  5^"  =«  ♦^  act 

^='J'Sg-£2'SE 


Soc 
£■5^-5; 


i  ki  d  u  ct 


AMMETERS.    VOLTMETERS.   AND    WATTMETERS         541 

Ones.  How  does  the  winding  differ  in  ammeters  and 
voltmeters  ? 

Ans.  An  ammeter  coil  consists  of  a  few  turns  of  heavy  wire 
(when  designed  to  carry  the  full  current),  while  a  voltmeter  coil 
is  wound  with  many  turns  of  fine  wire.  Thus,  the  ammeter  is  of 
low  resistance,  and  the  voltmeter  of  high  resistance. 


Fig.  623. — New  moving  element  of  Keystone  instruments,  weight  1.2  grams. 

Fig.  624. — Mo\'ing  element  of  Keystone  instruments  assembled  in  bearing.  The  moving 
element  consists  of  coil,  counterpoise  and  pointer.  The  mechanical  connections  are  made 
by  means  of  screws  and  steady  pins.  In  order  to  adjust  for  slight  set  or  subset  of  spring 
under  long  use  a  zero  adjuster  is  provided  by  means  of  which  this  set  can  be  connected 
and  the  pointer  brought  back  to  zero. 


542  HAWKINS  ELECTRICITY 

Ques.  Why  is  a  high  resistance  coil  used  with  a  volt- 
meter? 

Ans.  As  actuall}'^  constructed,  most  voltmeters  are  simply 
special  forms  of  ammeter.  From  Ohm's  law,  the  current  through 
a  given  circuit  equals  the  pressure  at  its  terminals  di\'ided  by  its 
resistance.  Hence,  if  a  high  resistance  be  connected  in  series 
with  a  sensitive  ammeter  that  will  measure  ven.'  small  currents, 
then  the  current  passing  through  the  circuit  is  directly  proportional 
to  the  voltage  at  its  terminals,  and  the  instrument  may  be  cal- 
ibrated to  read  volts. 


Figs.  625  and  626.- 

such  that  ail  the  currr- 
where  the  instrumer.:  . 

across  a  shtmt.as  in  f.„ _  __. — ._  ^;-_--. 

instrament  and  the  remainder  iixrou^  ihe  aauiii 


Ques.     Into  what  two  classes  may  ammeters  be  divided  ? 

Ans.  They  are  classed  as  series  or  shunt  according  to  the  way 
they  are  designed  to  be  connected  with  the  circmt. 

Ques.  What  determines  the  mode  of  connecting  am- 
meters? 

Ans.  When  the  wire  of  the  ammeter  coil  is  large  enough  to 
carry  the  whole  current,  it  is  coimected  in  the  circuit  in  series 
as  shown  in  fig.  625.  If,  however,  the  wire  be  small,  the  instru- 
ment is  connected  in  parallel  with  a  shunt  of  low  resistance,  so 
that  it  only  carries  a  small  part  of  the  current,  as  in  fig.  626. 


AMMETERS.    VOLTMETERS,  AND   WATTMETERS         543 

For  circuits  which  carry  large  currents,  the  shunt  connection  is  always 
used,  because  otherwise  the  coil  of  the  ammeter  would  have  to  be  very 
heavy  and  the  instrument  correspondingly  bulky. 

Oues.  How  are  shunt  ammeters  arranged  to  correctly 
measure  the  current? 

Ans.  The  coil  is  arranged  so  that  a  definite  proportion  of  the 
whole  current  passes  through  it.  A  large  conductor  of  low  re- 
sistance is  connected  directly  between  the  two  terminals  or  bind- 
ing posts  of  the  instrument;  the  coil  is  connected  as  a  shunt 
around  a  definite  part  of  this  main  conductor;  then,  since  the 
two  axe  connected  in  parallel  and  each  branch  has  a  definite 


&■'  IJl[.l  ^ 


1,000  AMPhKh;  Type  B  Shunt  400  Ampere  Type  D  Shunt 

Figs.  627  and  628. — Westinghouse  ammeter  shunts.  These  shunts  are  used  where  heavy 
currents  are  to  be  measured.  The  shunt  is  connected  in  series  with  the  bus  bar  or  circuit 
to  be  measured,  and  its  terminals  are  connected  by  means  of  small  leads  to  the  ammeter 
or  other  instrument.  These  shunts  are  designed  to  have  approximately  50  millivolts  drop 
at  full  rated  current.  They  are  intended  primarily  for  Westinghouse  meters,  but  can  be 
used  satisfactorily  with  any  meter  requiring  50  millivolts  for  full  scale  deflection. 


resistance,  the  current  divides  between  the  two  branches  directly 
in  proportion  to  their  relative  conductivities,  or  inversely  ac- 
cording to  their  resistances.  The  coil,  therefore,  takes  a  defi- 
nite part  of  the  whole  current,  and  the  force  moving  it  and  its 
pointer  away  from  the  zero  position  is  directly  proportional  to 
the  whole  current.  Hence,  by  providing  a  proper  scale,  the  value 
of  the  entire  current  will  be  indicated. 

Oues.    How  is  a  voltmeter  connected  ? 

Ans.     A  voltmeter  is  always  connected  to  the  two  points, 
whose  difference  of  potential  is  to  be  measured. 


544  HAWKINS  ELECTRICITY 


For  instance,  to  measure  the  voltage  between  the  two  sides  A  and  B 
of  the  circuit  shown  in  fig.  629,  one  terminal  of  the  voltmeter  is  connected 
to  wire  A,  and  the  other  to  wire  B.  If  the  "  drop  "  or  difference  in  voltage 
through  a  certain  length  of  wire  L,  of  a  circuit,  as  from  A  t-o  B  in  fig.  630 
is  to  be  determined,  one  terminal  of  the  voltmeter  is  connected  to  A  and 
the  other  to  B.    In  a  similar  manner  is  found  the  drop  through  a  lamp. 

Ques.  What  is  the  difference  between  a  voltmeter  and 
an  ammeter? 

Ans.  A  voltmeter  measures  pressure,  while  an  animeter 
measures  current.  As  actually  constructed,  most  voltmeters 
are  simply  special  forms  of  ammeter. 


a 


B 


Fig.  629. — ^Voltmeter  connection  for  measuring  the  pressure  in  an  electric  circuit.  The  volt- 
meter is  connected  in  parallel  in  the  circuit  at  the  point  where  the  voltage  is  to  be  meas- 
ured. 

Fig.  630. — Voltmeter  connection  for  measuring  the  "drop"  or  fall  in  voltage  in  a  certain  length 
of  wire,  as  for  instance,  the  length  between  the  points  A  and  B.  The  voltmeter  is  shuttled 
between  the  two  points  whose  pressure  difference  is  to  be  measured. 


If  a  high  resistance  be  connected  in  series  with  a  sensitive  ammeter 
that  will  measure  very  small  currents,  then  the  current  passing  through 
the  circuit  is  directly  proportional  to  the  pressure  or  voltage  at  its 
terminals  and  the  instrument  may  be  calibrated  to  read  volts. 

Ques.     Explain  the  term  "  calibrate." 

Ans.  To  calibrate  a  measuring  instrument  is  to  determine 
the  variations  in  its  readings  by  making  special  measiu-ements, 
or  by  comparison  with  a  standard. 

Ques.     Describe  a  solenoid  or  plunger  ammeter. 

Ans.  This  type  consists  of  a  "plunger"  or  soft  iron  core 
arranged  to  enter  a  solenoid.  Current  being  passed  through  the 
wire  of  the  solenoid  causes  the  core  to  be  more  or  less  attracted 


AMMETERS.    VOLTMETERS,  AND   WATTMETERS         545 


Fr.. 


TO  LOWER  SPRING -^  nO  UPPER  5PRm6 

631. — Weston  ammeter;  view  showing  shunt  enclosed  within  the  instrument.  West  n 
instruments  are  direct  reading  and  dead  beat.  Although  the  scales  have  practically  uniform 
divisions,  it  is  not  assumed  in  the  calibration  that  they  are  uniform,  and  the  scales  are  not 
printed  or  engraved.  The  method  of  calibration  consists  in  laying  out  each  large  division 
of  the  scale  bycomparing  the  instrument  with  a  standard,  and  then  inking  in  the  division 
lines  so  found.  The  smaller  divisions  between  the  large  ones  are  then  equally  spaced  and 
marked  by  a  mechanical  method. 


KiG.  632.— Weston  Portable  voltmeter,  inspector's  style.  This  instrument  is  provided  with  a 
reversmg  Icey.  Instead  of  the  regular  binding  posts,  pins  are  used  with  which  connections 
are  .made  by  means  of  contact  cups  attached  to  flexible  cords.  These  contact  cups  are 
convenient  m  making  connections,  or  in  changing  quickly  from  one  range  to  the  other,  if 
ine  mstrument  have  a  double  scale.  Connections  for  the  different  ranges  are  made  in  pre- 
cisely the  same  way  as  with  the  regular  double  scale  voltmeters.  For  the  upper  scale 
values,  the  contact  pm  to  the  right  and  the  front  contact  pin  to  the  left  being  taken,  and 
lor  the  lower  scale  values,  the  left  contact  cup  is  changed  to  the  rear  contact  pin. 


546 


HAWKINS  ELECTRICITY 


against  a  restraining  force  of  gravity  or  springs.  A  pivoted 
pointer  attached  to  the  core  indicates  the  current  value  on  a 
graduated  dial  as  shown  in  fig.  633. 

Ques.    What  are  the  objections  to  plunger  instruments? 

Ans.     They  are  not  reliable  for  small  readings,  and  are  readily 
affected  by  magnetic  fields. 


•WEIGtrr 


PLUNGER 


SOLENOID 


Fig.  633. — Plunger  type  instrument.  The  current  to  be  measured  passes  through  the  solenoid, 
producing  a  magnetic  effect  on  the  soft  iron  plunger  which  tends  to  draw  it  into  the  coil, 
and  thus  cause  the  pointer  to  move  over  the  graduated  scale.  The  distance  the  rod 
moves  depends  on  the  value  of  the  restraining  force  (which  may  be  springs  or  gravity), 
the  coil  winding,  and  strength  of  current.  The  winding  consists  of  a  few  turns  of  heavy 
wire  for  an  ammeter,  and  a  large  number  of  turns  of  fine  wire  when  constructed  as  a  volt- 
meter. Since  the  iron  has  a  certain  amount  of  residual  magnetism,  the  deflection  with 
smaller  following  large  currents  is  more  than  would  be  produced  by  the  same  current 
following  a  smaller  one.    The  instmment  therefore  is  less  reliable  than  the  usual  types. 


Ques.    Describe  a  magnetic  vane  instrument. 

Ans.  It  consists  of  a  small  piece  of  soft  iron  or  vane  mounted 
on  a  shaft  that  is  pivoted  a  little  off  the  center  of  a  coil  as  shown 
in  fig.  634.     The  principle  upon  which  the  instrument  works  is 


AMMETERS.    VOLTMETERS,  AND   WATTMETERS        547 

that  a  piece  of  soft  iron  placed  in  a  magnetic  field  and  free  to 
move  will  move  into  such  position  as  to  conduct  the  maximum 
number  of  lines  of  force.  The  current  to  be  measured  is  passed 
around  the  coil  producing  a  magnetic  field  through  the  center 
of  the  coil.  The  magnetic  field  inside  the  coil  is  strongest  near 
the  inner  edge,  hence,  the  vane  will  move  against  the  restraining 
force  of  a  spring  so  that  the  distance  between  it  and  the  inner 
edge  of  the  coil  will  be  as  small  as  possible.  A  pointer,  attached 
to  the  vane  shaft  moves  over  a  graduated  dial. 

•SCALE 

•POINTER 

SPRING 

'  VANE 
OIL 


Fig.  634. — Magnetic  vane  instrument.  A  soft  iron  vane,  eccentrically  pivoted  within  a  coil 
carrying  the  current  to  be  measured,  is  attracted  toward  the  position  where  it  will  con- 
duct the  greatest  number  of  magnetic  lines  of  force  against  the  restraining  force  of  a  spring 
or  equivalent. 


Ques.     Describe  an  inclined  coil  instrument. 

Ans.  As  shown  in  fig.  635,  a  coil  carrying  the  current,  is 
mounted  at  an  angle  to  a  shaft  to  which  is  attached  a  pointer. 
A  bundle  of  iron  strips  is  mounted  on  the  shaft.  A  spring  re- 
strains the  shaft  and  holds  the  pointer  at  the  zero  position  when 
no  cturent  is  flowing.    When  a  cvirrent  is  passed  through  the  coil, 


548 


HAWKINS  ELECTRICITY 


the  iron  tends  to  take  up  a  position  with  its  longest  sides  parallel 
to  the  lines  of  force,  which  results  in  the  shaft  being  rotated  and 
the  pointer  moved  on  the  dial,  the  amount  of  movement  de- 
pending upon  the  strength  of  the  current  in  the  coil. 

The  coils  for  large  sizes  are  generally  wound  with  a  few  turns  of  flat 
insulated  copper  ribbon.  The  instruments  are  adapted  to  either  direct 
or  alternating  currents  but  are  recommended  for  alternating  currents. 

Oues.     W  hat  is  the  principle  of  the  hot  wire  instrument? 

Ans.     Its  action  depends  upon  the  heating  of  a  conductor  by 
the  current  flowing  through  it,  causing  it  to  expand  and  move  an 


POIMTER^ 


SPRING 


iSCAl-E. 


MAGNETIC 
VANE 


CO\L 


Fig.  635. — Thompson  inclined  coil  ammeter.  It  is  constructed  on  the  magnetic  vane  principle 
in  which  an  iron  vane  is  attracted  by  the  magnetic  field  due  to  the  coil,  so  as  to  turn  itself 
parallel  with  the  axis  of  the  coil,  the  latter  being  inclined  with  respect  to  the  axis  of  the  vane. 
The  voltmeter  of  this  type  has  a  similarly  placed  stationary  coil,  but  in  place  of  the  iron 
vane,  is  pro%-ided  with  a  mo%-ing  coil  in  series  with  the  other  coil.  The  restraining  force  in 
each  case  being  that  due  to  springs.  Figs.  636  and  637  show  the  actual  construction  of 
inclined  coil  instruments. 


index  needle  or  pointer,  the  movements  of  which,  by  calibration, 
are  made  to  correspond  to  the  pressure  differences  producing 
the  actuating  ctirrents. 


Oues. 
ments? 


What  are  the  characteristics  of  hot  wire  instru- 


Ans.     Voltmeters  of  this  type  are  not  affected  by  magnetic 
fields,  and  as  their  self-induction  is  small,  they  can  be  used  on 


AMMETERS,    VOLTMETERS,  AND   WATTMETERS         549 


either  direct  or  alternating  currents;  but  they  possess  certain 
serious  defects :  they  consume  more  current  than  the  other  types ; 
cannot  be  constructed  for  small  readings;  are  liable  to  burn  out 
on  accidental  overloads ;  and  are  somewhat  vague  in  the  readings 
near  the  zero  point  and  are  sometimes  inaccurate  in  the  upper 
part  of  the  scale. 


Figs.  636  and  637. — Thompson  inclined  coil  portable  indicating  instruments.  Pig.  636,  am- 
n^eter  type  P  interior;  fig.  637,  wattmeter,  type  P,  interior.  These  instruments,  though 
primarily  designed  for  use  on  alternating  current  circuits,  may  also  be  used  on  direct 
current  circuits,  by  making  reversed  readings  and  taking  the  mean  as  the  true  indication. 
The  voltmeters  and  wattmeters  are  constructed  on  the  dynamometer  principle  and  the 
ammeters,  on  the  magnetic  vane  principle.  The  voltmeters  and  wattmeters  are  provided 
with  a  contact  key  which  may  be  locked  in  position,  enabling  the  instruments  to  be  left 
conscantly  in  circuit.  The  movements  of  the  pointer  are  damped  by  means  of  an  air  vane; 
the' 3  is  also  a  friction  damping  device  operated  by  a  small  button  to  check  excessive 
osc  llations  of  the  pointer.  The  inclined  coil  instrument?  are  so  designed  that  the  torque 
is  ufficiently  high  to  insure  the  pointer  assuming  a  definite  position  with  each  change  in 
cuixent  value. 

Oues.  Describe  the  construction  and  operation  of  the 
Whitney  hot  wire  instruments. 

Ans.  As  shown  in  fig.  638,  a  wire  AX,  of  non-oxidizable  metal, 
of  high  resistance  and  low  temperature  coefficient,  passes  over  a 
pulley  B  mounted  on  the  shaft  C.  The  ends  of  the  wire  are 
attached  to  the  plate  E  at  its  ends  F  and  G,  the  wire  being  in- 
sulated from  the  plate  at  G.  A  spring  H  holds  the  wire  in  tension 
and  takes  up  the  slack  due  to  the  expansion  caused  by  the  heat- 
ing of  the  wire  when  a  current  passes  through  it.    The  current 


550 


//.4irA'/.V5  ELECTRICITY 


flows  only  in  the  portion  of  the  wire  marked  A,  between  the  plate 
E  and  the  pulley  B  up  to  the  point  K  where  the  connection  is 
shown.  When  a  cturent  flows  through  the  wire  A,  the  spring 
takes  up  the  slack,  pulls  A  around  B,  and  causes  B  to  rotate  upon 
its  shaft  C.  It  is  clear,  that  a  pointer  attached  to  C,  would  indi- 
cate on  a  scale  the  movement  of  B  and  C,  but  as  this  movement 
is  very  slight,  a  magnifpng  de\-ice  will  be  required.    This  de\'ice 


Fig.  638. — piagram  showing  principle  and  construction  of  the  Whitney  hot  wire  instruments. 
The  action  of  instruments  of  this  type  depends  on  the  heating  of  a  wire  by  the  passage  ot 
a  current  causing  the  wire  to  lengthen.  This  elongation  is  magnified  by  suitable  mechan- 
ism  and  transmitted  to  the  pointer  of  the  instrument. 

consists  of  a  forked  rod  L,  rigidly  attached  to  the  shaft  C,  and 
carrying  at  its  lower  end  a  silk  fibre  fastened  to  the  fork  and  passing 
around  a  pulley  M,  to  which  a  pointer  N  is  attached.  For  direct 
current  measurements  only  an  electromagnetic  system  is  used. 

Oues.     What  is   the  principle  of   electrostatic  instru- 
ments? 

Ans.     The  action  of  these  instruments  depends  upon  the  fact 
that  two  conductors  attract  one  another  when  any  difference  of 


AMMETERS,    VOLTMETERS,  AND    WATTMETERS         551 


electric  pressure  exists  between  them.     If  one  be  delicately  sus- 
pended so  as  to  be  free  to  move,  it  will  approach  the  other. 

Oues.    Describe  the  Kelvin  electrostatic  voltmeter. 

Ans.  A  simple  form  consists,  as  shown  in  fig.  639,  of  a  metal 
case  containing  a  pair  of  highly  insulated  plates,  between  which 
a  deHcately  mounted  paddle  shaped  needle  is  free  to  move.  When 
the  needle  is  connected  to  one  side  of  a  circuit  and  the  stationary 


Fic.  639. — Kelvin  electrostatic  voltmeter;  a  form  of  instrument  designed  for  measuring  high 
pressures  up  to  200,000  volts.  The  instrument,  as  illustrated,  consists  of  fixed  and  movable 
vanes  with  terminals  connecting  with  each.  These  vanes  which  act  as  condensers  take 
charges  proportional  to  the  potential  difference  between  them,  resulting  in  a  certain  attrac- 
tion which  tends  to  rotate  the  movable  disc  against  the  restraining  force  of  gravity.  In 
the  figure  aa  and  b  are  two  fixed  vanes  and  c  a  movable  vane,  carrymg  a  pointer  and  hav- 
ing a  proper  weight  at  its  lower  end. 

plates  to  the  other  side,  the  needle  is  attracted  and  moves  be- 
tween them  as  indicated  by  the  pointer.  Adjusting  screws  at  the 
lower  end  of  the  needle  allow  it  to  be  balanced  so  that  its  center 
of  gravity  is  somewhat  below  the  center  of  suspension.  Gravity 
then  is  the  restraining  force. 


552  HAWKINS  ELECTRICITY 


The  range  of  the  instrument  may  be  changed  by  hanging  different 
weights  upon  the  needle.  B3'  increasing  the  number  of  blades  the  in- 
strument can  be  made  to  measure  as  low  as  30  volts.  The  form  having 
two  stationary  blades  and  one  movable  blade  is  suitable  for  measuring 
from  200  to  20,000  volts.  The  quadrant  electrometer  or  laboratory  form 
will  measure  a  fraction  of  a  volt. 


Fig.  &40. — Thompson  astatic  instniment  without  cover.  When  cairrent  passes  through  the 
coils  of  the  moving  element,  the  lines  of  force  parallel  to  the  shaft  produce  a  torque  which 
tends  to  turn  the  shaft  and  caxise  the  needle  to  travel  across  the  scale.  This  action  is.  of 
course,  opposed  by  the  magnetic  field  at  right  angles  to  the  shaft  acting  on  the  two  pieces 
o!  magnetic  metal.  These  astatic  instruments  have  no  controlling  springs.  The  two  small 
silver  spirals  which  conduct  the  current  to  and  from  the  armature  are  made  of  untempered 
silver  and  exert  no  force  as  springs.  The  actuating  and  restraining  forces  are  dependent 
upon  the  same  electromagnets.  The  dampring  effect  in  these  instruments  is  produced  by 
an  aluminum  disc  mo\'ing  in  a  magnetic  field,  and  is  proportional  to  the  square  of  the  loag- 
net  strength. 


Ques.  Explain  the  construction  and  principle  of  the 
Thompson  astatic  instruments. 

Ans.  The  fields  of  these  instruments  are  electromagnets 
wotmd  for  any  specified  voltage  and  provided  with  binding  posts 
separate  from  the  current  posts  of  the  instrument.  The  moving 
coils  are  mounted  upon  an  aluminum  disc  and  are  located  in  a 
magnetic  field  which  is  parallel  to  the  shaft  and  astatically 
arranged.     Two  small  pieces  of  magnetic  metal   are  rigidly 


AMMETERS,    VOLTMETERS.  AND    WATTMETERS        553 

mounted  on  the  shaft  and  the  astatic  components  of  the  magnetic 
field,  which  are  perpendicular  to  the  shaft,  tend  to  keep  the  pieces 
of  magnetic  metal  in  their  initial  positions.  When  current  passes 
through  the  coils  of  the  moving  element,  the  lines  of  force 
parallel  to  the  shaft  produce  a  torque  which  tends  to  turn  the 
'ihaft  and  cause  the  needle  to  travel  across  the  scale.    This  action 


Fig.  041  to  642.— Multipliers  for  Weston  standard  portable  voltmeters.  Multipliers  are  re- 
sistance bo.xes,  the  coils  in  which  are  highly  insulated,  and  are  adjusted  so  that  the  readings 
of  the  instrument  may  be  multiplied  by  any  desired  constant.  Multipliers  are  usually 
constructed  so  that  the  indications  of  the  pointer,  multiplied  by  2,  .5,  10,  20  or  .50,  will 
give  the  voltage  of  the  circuit.  By  the  use  of  multipliers  the  range  of  voltmeters  may  be 
increased  to  any  practical  limit. 


is,  of  course,  opposed  by  the  magnetic  field  at  right  angles  to  the 
shaft  acting  on  the  two  pieces  of  magnetic  metal.  There  are  thus 
no  restraining  springs,  current  being  conveyed  to  the  moving 
coil  by  tortionless  spirals  of  silver  wire.  Thompson  astatic 
instruments  can  be  provided  with  polarity  indicators,  a  red 
disc  showing  on  the  scale  card  where  the  ooles  are  reversed. 


554 


HAWKINS  ELECTRICITY 


The  effect  of  external  fields  is  eliminated  by  the  astatic 
arrangement  of  the  fields  and  the  moving  parts.  A  field  which 
tends  to  increase  the  torque  on  one  side  of  the  armature  dimin- 
ishes it  to  a  corresponding  degree  on  the  other  side.  The 
damping  effect  in  these  instruments  is  produced  by  an  aluminum 
disc  moving  in  a  magnetic  field. 


Fig.  643. — Portable  multiplier  for  portable  voltmeter.  A  multiplier  is  used  for  increasing  the 
readings  of  voltmeters,  and  consists  of  resis.ance  coils  placed  in  a  portable  case.  A  mul- 
tiplier is  connected  in  series  vrith  the  voltmeter  and  must  be  adjusted  for  the  instrument 
with  which  it  is  to  be  used,  because  the  resistance  coil  must  be  a  multiple  of  the  voltmeter 
resistance.  For  instance,  a  multiplier  wich  a  value  of  10,  used  with  a  6  volt  voltmeter  or 
521  ohms  would  measure  about  5.215  ohms;  one  with  a  value  of  40.  would  equal  about 
20,860  ohms.  The  multiplier  10  would  give  a  total  scale  value  of  60.  and  the  multiplier 
40,  a  total  scale  value  of  240  volts  to  the  6  volt  instrument.  A  multiplier  is  of  consider- 
able value  in  that  it  does  away  with  the  necessity  of  having  a  number  of  voltmeters  of 
different  ranges.    The  instrument  here  illustrated  has  a  range  of  150  volts, 

Ques.    What  are  multipliers? 

Ans.  These  are  extra  resistance  coils  which  are  connected  in 
series  wnth  a  voltmeter  for  increasing  its  capacity  or  readings. 
They  are  put  up  in  portable  boxes,  and  must  be  adjusted  for 
each  particular  voltmeter  as  the  resistance  of  a  multiplier  coil 
must  be  a  multiple  of  the  resistance  of  the  voltmeter  itself. 


AMMETERS.   VOLTMETERS,  AND   WATTMETERS         555 

Ques.     What  is  an  electro-dynamometer? 

Ans.  An  instrument  for  measuring  amperes,  volts,  or  watts 
by  the  reaction  between  two  coils  when  the  current  to  be 
measured  is  passed  through  them.  One  of  the  coils  is  fixed  and 
the  other  movable. 

Ques.     Descrfbe  the  Siemens'  electro-dynamometer. 

Ans.  The  essential  parts  are  shown  in  fig.  646.  The  fixed 
coil  A,  composed  of  a  number  of  turns  of  wire  is  fastened  to  a 
v^ertical  support,  and  surrounded  by  the  movable  coil  B  of  a  few 


Figs.  644  to  645. — Western  standard  portable  shunts.  The  milli-voltmeters  used  in  connection 
with  these  shunts  read  directly  in  amperes.  Shunts  of  different  capacities  can  be  adjusted 
to  the  same  instrument,  and  it  can,  therefore,  be  used  to  measure  a  current  of  2,000  amperes 
with  the  same  degree  of  accuracy  as  a  current  of  1  ampere.  In  selecting  shunts  of  different 
capacities  for  use  in  connection  with  one  instrument  it  should  be  considered  that  the  higher 
ranges  must  be  even  multiples  of  the  lower  one  in  order  to  suit  the  same  scale  on  the 
instrument. 

turns,  or  often  of  only  one  turn.  The  movable  coil  is  suspended 
by  a  thread  and  a  spiral  spring  C,  below  the  dials  which  are 
fastened  at  one  end  to  the  movable  coil  and  at  the  other  end  to  a 
milled  headed  screw  D,  which  can  be  turned  so  as  to  place  the 
planes  of  the  coil  at  right  angles  to  each  other,  and  to  apply 
torsion  to  the  spring  to  oppose  the  deflection  of  the  movable  coil 
for  this  position  when  a  current  is  passed  through  the  coils.    The 


556 


HAWKINS  ELECTRICITY 


ends  of  the  movable  coil  dip  into  two  cups  of  mercury  E,  E', 
located  one  above  the  other  and  along  the  axis  of  the  coils  so 
as  to  bring  the  two  in  series  when  connected  to  an  external 
circuit.  The  arrows  show  the  direction  of  current  through  the 
two  coils.  An  index  pointer  F  is  attached  to  the  movable  coil. 
The  upper  end  of  this  pointer  is  bent  at  a  right  angle,  so  that  it 


Fig.  C40. — Diagram  of  Siemens'  electro-dynamometer.  It  consists  of  two  coils  on  a  common 
axis,  but  set  in  planes  at  right  angles  to  each  other  in  such  a  way  that  a  torque  is  produced 
between  the  two  coils  which  measures  the  product  of  their  currents.  This  torque  is  balanced 
by  twisting  a  spiral  spring  through  a  measured  angle  of  such  degree  that  the  coils  shall 
resume  their  original  relative  positions.  If  the  instrument  be  used  for  measuring  current, 
the  coils  are  connected  in  series,  and  the  reading  is  then  proportional  to  the  square  of  the 
current.  If  used  as  a  wattmeter,  one  coil  carries  the  main  current  and  the  other  a  small 
current,  which  is  proportional  to  the  pressure.  The  reading  is  then  proportional  to  the 
power  in  the  circuit. 

Fig.  647. — Diagram  showing  connections  of  Siemens'  electro-dynamometer  as  arranged  to 
read  watts. 


swings  over  the  dial  between  two  stop  pins  G,  G',  and  rests 
directly  over  the  zero  line  when  the  planes  of  the  coils  are  at 
right  angles  to  each  other.  A  pointer  H  is  attached  to  the  torsion 
screw  D,  and  sweeps  over  the  scale  of  the  dial.  The  spring  is  the 
controlling  factor  in  making  the  measurement 


AMMETERS,    VOLTMETERS,  AND    WATTMETERS         557 


Figs.  648  to  650. — Wright  demand  Indicator.  This  is  a  device  for  registering  the  maximum 
ampere  demand  of  appreciable  duration  in  any  electrical  circuit.  It  may  be  used  on  either 
direct  or  alternating  current  circuits.  The  essential  features  and  principle  are  as  follows: 
A  liouid  is  hermetically  sealed  in  a  glass  vessel  consisting  of  two  bulbs  connected  by  a 
"U"'  tube,  and  a  central  tube  called  the  "index"  tube,  connected  to  the  upper  end  of  the 
right  hand  side  of  the  "U."  Around  the  left  hand  or  heating  bulb,  is  placed  a  band  of 
resistance  metal,  through  which  the  current  to  be  measured  is  passed,  or  a  definite  shunted 
portion  of  it.  The  heating  effect  of  the  current  increases  the  temoerature  of  the  left  hand 
bulb,  causing  the  air  to  expand  which  forces  the  liquid  up  the  right  hand  side  of  the  "  U  " 
tube  and  into  the  index  tube,  where  it  remains  until  the  indicator  is  reset.  The  height  of 
the  liquid  in  the  index  tube  as  shownby  the  scale,  indicates  the  maximum  current  which 
has  passed  through  the  indicator.  It  is  the  difference  in  temperature  of  the  air  in  the  two 
bulbs  which  causes  the  flow  ol  the  liquid.  Any  change  in  e.xternal  temperature  causes 
equal  effect  in  both  bulbs  and  therefore  does  not  affect  the  reading. 


558 


HAWKINS  ELECTRICITY 


Ques.  Explain  the  operation  of  the  Siemen's  electro- 
dynamometer. 

Ans.  In  fig.  646,  when  a  current  is  passed  through  both  coils, 
the  movable  coil  is  deflected  against  a  stop  pin,  then  the  screw 
D  is  t-urned  in  a  direction  to  oppose  the  action  of  the  current  until 


Figs.  651  and  ti.')2. — Weston  illuminated  dial  station  voltmeter  and  an-.r:e:er.  The  voltmeter 
has  two  indices,  a  pointed  index  for  close  readings  and  an  index  called  the  normdl  indfx . 
which  enables  a  slight  deviation  from  the  normal  voltage  to  be  seen  from  a  long  distanci 
The  "normal  index  "  is  inside  the  case  and  terminates  in  a  circular  disc  of  blackened  alumi- 
num. The  disc  is  adjusted  from  the  outside  of  the  case  by  hand,  by  means  of  the  knurled 
knob  seen  on  the  front  of  the  case,  so  that  it  is  directly  below  the  point  of  normal  voltage. 
When  the  indicating  index  reaches  the  point  of  normal  voltage,  the  disc  of  the  normal 
index  appears  in  the  center  of  the  circular  opcming  of  the  indicating  index,  a  narrow  ring 
of  white  being  visible,  encircling  the  disc  of  the  normal  index.  The  ammeter  depends  for 
its  operation  upon  the  fall  of  potential  between  two  points  of  the  circuit  carr>-ing  the  main 
current,  and  requires  a  difference  of  only  about  .05  volt  to  give  a  full  scale  deflection.  When 
a  maximum  deflection  is  secured,  the  current  passing  through  the  instrument  is  never  more 
than  .07  ampere  irrespective  of  the  total  capacity  of  the  instrument.  A  separate  shunt  if 
used  which  is  placed  at  the  back  of  the  switchboard.  In  many  cases,  a  special  shunt  can 
be  dispensed  with  and  a  short  section  of  the  mains  on  the  switchboard,  or  the  mains  lead- 
ing from  the  djTiamo,  can  be  used  instead.  On  the  basis  of  one  square  inch  cross  section 
per  1.000  amperes,  a  length  of  about  5  feet  of  copper  conductor  would  be  required  as  a 
shunt,  in  which  case  however,  this  section  of  the  conductor  must  be  adjusted  with 
precision. 


the  deflection  has  been  overcome  and  the  coil  brought  back  to 
its  original  position.  The  angle  through  which  the  pointer  of  the 
torsion  screw  was  turned  is  directly  proportional  to  the  square 
root  of  the  angle  of  torsion.    To  determine  the  current  strength 


AMMETERS,    VOLTMETERS,  AND   WATTMETERS         559 

in  amperes,  the  square  root  of  the  angle  of  torsion  is  multipHed 
by  a  calculated  constant  furnished  by  the  makers  of  this  in- 
strument. 

Oues.  How  is  the  electrodynamometer  adapted  to 
measure  volts  or  watts? 

Ans.  When  constructed  as  a  voltmeter,  both  coils  are  wound 
with  a  large  number  of  turns  of  fine  wire  making  the  instrument 


Fig.  653. — Thompson  watt  hour  meter  (type  C-6).  This  form  is  furnished  with  side  con- 
nections, the  line  wires  entering  at  the  left  and  the  load  wires  at  the  right.  Both  sides  of 
the  system  are  carried  through  the  meter  in  all  sizes  up  to  and  including  the  50  ampere 
size.    In  meters  of  larger  ampere  capacities,  a  voltage  tap  is  used. 

sensitive  to  small  currents.  Then  by  connecting  a  high  resistance 
in  series  with  the  instrument  it  can  be  connected  across  the  ter- 
minals of  a  circuit  whose  voltage  is  to  be  measured.  When  con- 
structed as  a  wattmeter,  one  coil  is  wound  so  as  to  carry  the  main 


500 


HAWKIXS  ELECTRICITY 


current,  and  the  other  made  with  many  ttims  of  fine  vrire  of 
high  resistance  sviitable  for  connecting  across  the  ciraiit.  With 
this  arrangement,  the  force  between  the  two  coils  will  be  pro- 
portional to  the  product  of  amperes  by  volts,  hence,  the  in- 
strument will  meastu-e  watts. 


Pig.  6S4. — Interior  view  of  Thompson  watt  hoar  meter  (type  C-6).  Capacity:  5  to  600 
amperes,  two  wire,  and  5  to  300  amperes,  three  wire;  100  to  250  vtdts.  The  meter  is 
supported  by  three  logs,  the  tipper  one  of  which  is  keyboled.  and  the  lower  right  hand  one 
slotted.  This  permits  rapid  and  aocnrate  levdling  as  the  top  screw  can  be  inserted  and 
the  meter  hung  thereon  approximatdy  lereL  The  right  hand  screw  may  then  be  i^aced 
in  positiba  and  the  meter  adjusted  as  may  be  required  before  forcing  the  screw  hosne. 

Oues.  Describe  briefly  the  construction  of  the 
Thompson  recording  wattmeter. 

Ar.s.  It  consists  of  four  elcnienis:  1,  a  motor  causing  rota- 
tion; 2,  a  d\Tiamo  pro\-iding  the  necessarj'^  load  or  drag;  3,  a 
registering  de\-ice,  the  function  of  which  is  to  integrate  the 
instantaneous  values  of  the  electrical  energ>'  to  be  measured. 
and  4,  means  of  regulation  for  light  and  full  load- 


AMMETERS,  VOLTMETERS  AND  WATTMETERS  ^^[\\ 


iiiniiiiiiiirCT 


g;go„,j::o"^oi2oES«J 
^Sti^-I-'^.S  «io  E  fe  S  S 

«  a,  ''^  fc^  C^  g,b  S  «  rt 

«S  5"  Si^^^S^  «'-=  c^  ajS 
;S  o  M  (u  "*  <u^  S-«T^    ^0+3 

-«„  o  fc  ?;  aS^c  o  Sd  E  "  s-d 
0^1)  c  Ji-c:^!,-  e  p.'^  S  S  ^ 

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=     .HE 


3^  o 


C!   n  r?   u^  J3   C 


• — ^  '^  .^  ooiw  be  ^-*  «  —  (11  rl 
■"J^'^.S-^  1-  o  "i-^  S--  o^  o  J? 

2    .  i- ":    M^'^S      C  1/ <u  4)  t^ 

o  ^  °  o  °W  £  °  E:S  E  rt-3  t;*- 

"Sx^s-^i^  w  "  ^^"^  i;^  ^  °  °  2 

■•-*    C    tn"       O    f— '    O    ^    W   «- .X    I-    r3 

^>.|l^-2§|ES§|i-l 
^-^g^^u5^3->-E-S43 

S  gj=i!-?  2  3  "5  Mj;  C  3.C2  3 
«  nj  (u  M  i^--^j^^  «-C^  oj  u  rt  f^ 


562 


HAWKINS  ELECTRICITY 


Oues.  What  is  the  action  of  the  motor  in  the  Thomp- 
son watt  hour  meter? 

Ans.  It  rotates  at  very  slow  speed,  and  since  there  is  no 
iron  in  its  fields  and  armature,  it  has  very  little  reverse 
voltage.  Its  armature  current,  therefore,  is  independent  of 
the  speed  of  rotation,  and  is  constant  for  any  definite  voltage 
applied  at  its  termi-    '- 


Fig.  656. — Interior  of  Thompson  watt  hojir  meter  (type  C-6)  showing  armature,  small  commu- 
tator and  gravity  brushes.  A  spherical  armature  moving  within  circular  field  coils  is 
the  construction  adopted  in  this  meter.  The  armature  is  wound  on  a  very  thin  paper 
shell,  stiff  enough  to  withstand  the  strain  due  to  winding  and  subsequent  handling.  The 
wire  composing  the  armature  is  of  the  smallest  gauge  consistent  with  mechanical  strength. 
The  field  coils,  as  before  stated,  are  circular,  and  are  placed  as  near  each  other  as  possible, 
one  on  either  side  of  the  armature,  with  the  internal  diameter  just  sufficient  to  give  the 
necessar>'  clearance  for  the  rotating  element.  This  construction  prevents  magnetic  leak- 
age. Ribbon  wire  is  employed  for  the  field  coUs,  thus  economizing  space  and  further 
carrying  out  the  idea  of  concentration. 

The  torque  of  this  motor  being  proportioned  to  the  product  of  its 
armature  and  field  currents,  must  vary  directly  as  the  energy  passing 
through  its  coils.  In  order  then,  that  the  motor  shall  record  correctly  it 
is  necessary  only  to  provide  some  means  for  making  the  speed  propor- 
tional to  the  torque.  This  is  accomplished  by  applying  a  load  or  drag, 
the  strength  of  which  varies  directly  as  the  speed- 


AMMETERS,    VOLTMETERS  AND    WATTMETERS  563 

Oues.     Explain  the  operation  of  the  Thompson  record- 
ing wattmeter. 

Ans.  There  being  no  iron  in  either  field  or  armature  of  the 
motor  element,  no  considerations  of  saturation  are  involved. 
The  torque  or  pull  of  the  armature  is  dependent  upon  the  product 
of  the  field  and  armature  strength.  The  strength  of  the  field — 
there  being  no  iron — varies  directly  with  the  current  in  the  field. 
Thus  the  strength  of  the  field  with  10  amperes  flowing  to  the  load 
is  exactly  twice  the  strength  of  the  field  with  5  amperes  flowing 
to  the  load.  The  strength  of  the  armature  is  dependent  on  the 
voltage  of  the  system  to  which  it  is  connected,  the  armature 
eleinent  of  the  meter  being  practically  a  voltmeter.  There  is, . 
therefore,  a  torque  br  pull  varying  directly  with  the  strength  of 
the  armature  multiplied  by  the  strength  of  the  field,  or,  in  other 
words,  varying  directly  with  the  watt  load,  and  except  in  so  far 
as  influenced  by  friction,  the  speed  of  rotation  varies  directly 
with  the  torque  or  pull.  The  currents  generated  in  the  disc 
armature  consist  of  eddy  currents,  which  circulate  within  the 
mass  of  the  disc. 


Installation  of  Wattmeters. — The  various  types  of  watt- 
meter differ  so  widely  either  in  mechanical  details,  or  operating 
principles,  that  it  is  customary  for  manufacturers  to  furnish 
detailed  instructions  for  the  installation  of  their  meters.  Such 
instructions  should  be  carefully  followed  in  all  cases,  but  the 
following  will  be  found  generally  applicable  to  all  types  of 
motor  meter : 

1.  After  unpacking  the  meter,  and  before  opening  the  case  or  cover, 
clean  the  latter  carefully  to  remove  all  adhering  particles  of  dust 
and  excelsior. 

2.  The  proper  location  for  the  meter  should  be  one  where  there  is  no 
vibration.  When  this  location  has  been  selected,  nail  or  screw  upon 
the  walls,  a  board  somewhat  larger  than  the  dimensions  of  the  back 
of  the  meter,  and  upon  this  board  hang  the  meter  by  the  top  hanger. 


564  HAWKINS  ELECTRICITY 


3.  After  hanging  the  meter,  open  or  remove  the  case  or  cover,  and  if 
necessar}',  put  the  mechanism  in  order  according  to  instructions 
furnished  by  the  manufacturer. 

4.  In  order  to  operate  satisfactorily,  the  meter  should  hang  plumb,  so 
that  the  spindle  of  the  revolving  element  will  be  vertical,  and  the 
horizontal  planes  through  the  armature  and  retarding  disc  will  be 
level.  Many  complaints  relative  to  meters  being  slow  on  light  loads, 
are  invariably  due  to  the  fact  that  the  meters  have  been  installed 
out  of  plumb.* 


Fig.  657. — Interior  view  of  Thompson  watt  hour  meter  (t>T)e  CQ).  The  capacities  of  this 
type  raijge  from  60  to  400  amperes  inclusive,  two  wire,  and  50  to  200  ami>eres  inclusive, 
three  wire,  and  for  voltages  of  from  1 W)  to  600  volts.  These  meters  are  miade  with  either 
front  or  back  connections.  In  front  connected  meters  the  positions  of  the  leading-in  wires 
and  cables  are  the  same  as  in  the  type  C-6,  fig.  654,  so  that  either  type  of  meter  may  be 
installed  in  the  same  location. 


•XOTE. — The  most  practical  and  accurate  method  of  plumbing  a  meter  is  to  level  it  by 
meansof  a  small  brass  weight  placed  upon  the  retarding  disc.  Place  the  weight  upon  the  front 
or  back  upper  surface  of  the  disc,  close  to  the  edge.  If  the  disc  and  weight  rotate  toward  the 
right,  move  the  bottom  of  the  meter  in  the  same  direction  so  as  to  raise  the  disc  on  the  right. 
When  the  disc  is  level,  the  weight  and  disc  will  remain  stationary  when  the  weight  is  placed 
on  either  the  front  or  the  back  of  the  disc.  Xe.^t .  place  the  weight  on  the  disc  close  to  the  edge 
on  either  side.  If  the  disc  rotate  towards  the  front,  swing  the  bottom  of  the  meter  away  from 
the  wall  or  board  until  the  disc  remains  stationary'  when  the  weight  is  placed  upon  it  on  either 
side.  If  the  disc  rotate  toward  the  back,  raise  it  up  on  that  side  by  bringing  the  top  of  the 
meter  away  from  the  wall  or  board.  It  is  p>ossible  that  the  second  levelling  operation  will  alter 
the  position  of  the  disc  obtained  by  the  first  operation,  therefore,  the  first  should  be  repeated, 
and  after  that  the  second  also,  until  the  disc  remains  stationary  when  the  weight  is  placed  at 
any  point  upon  its  surface.  This  method  of  levelling  is  more  reliable  than  any  method  in 
which  a  spirit  level  is  employed. 


AMMETERS,  VOLTMETERS,  AND  WATTMETERS 


565 


taxi  r^^  c_  a"<-'"-= 
>>H^  M  c  o  °£^-^ 

_-d  cnB  bo=«  "S'^lc'^ji 

rt  "1  4>^  >^  O  o  -     .3 

•doi.o'i3ou"rt  Q.T3  •;: 

O  S^.-S  o-C  5  >  «>      _o 
T3  fto)  c  0!  -^  H  ""-^  2J 

o  S  aEi^  c^^icc'S  fi 
2^  o  >-  »«  c-Sp  5  fe.g 

-^ 5  tD^  o-°  >a  a 

5  u  1,  rt  o^  o  rt  oj::  c 
"■  2  2^  4,  o  u  j;  ^-d^ 
—  §•  o  njr-'_iJ  ^  m  .*"  5j 
0.2  ..  <D  C  rt  «  rt'C  S 

r;j=  S?  c  p  iM-e  "^.t;  «^  a> 


O  ■>->  o. 

„a..<2^oE^g 

c-r  o  C-G  o  "  ,n  m  ^-ti 

g  o£j=J5x  5  c  p  S  H 


56(i 


HAWKINS  ELECTRICITY 


UPPER  BEARING, 
SLEEVE  \ 


n 


DIAL- 


LIGHT  LOAD 

ADJUSTING 

COIL 


DAMPING 
DISK 


FRONT 

RETARDING 

MAGNETS 


SERIES 
^.-^COILS 
TERMINALS 

CLAMPING 
SCREW  B 

TERMINAL 


REAR 

RETARDING 
MAGNETS 


JEWEL  SCREW 


Fig.  f)59. — Westinghouse  type  CW-6  watt  hour  meter  with  cover  off.  This  meter  is  of  the 
commutator  type  without  iron  in  the  magnetic  circuit.  The  spherical  armature  is  closely 
surrounded  by  circular  field  coils  which  provide  the  shortest  magnetic  path  and  smallest 
magnetic  leakage,  thus  securing  high  torque  with  small  consumption  of  energ>'.  The 
armature  ivinding  is  wound  on  a  hollow  sphere  of  prepared  paper  which  is  moulded  in 
corrugated  form  to  secure  strength.  Uniform  brush  tension  is  m.aintained  by  gravity. 
Each  brush  consists  of  two  small  round  wires  placed  side  by  side  and  held  against  the  com- 
mutator by  a  small  counterweight  whose  distance  from  the  fulcrum  is  adjustable.  The 
current  winding  consists  of  two  flat  coils  of  strap  copper,  one  clamped  rigidly  on  either  side 
of  the  central  mounting  frame  which  supports  the  armature  bearings.  These  coils  are  con- 
nected either  in  series  or  parallel,  depending  on  the  capacity.  In  three  wire  meters  one  of 
the  coils  is  connected  in  series  with  each  side  of  the  line.  The  retarding  element  consists 
of  a  light  aluminum  disc  rotating  between  two  pairs  of  permanent  magnets.  The  magnets 
are  prepared  by  a  special  aging  process  to  insure  permanence.  Full  load  adjustment  is 
made  by  shifting  the  position  of  the  permanent  magnets.  Ample  light  load  adjustment  or 
friction  compensation  is  provided  by  means  of  the  movable  coil,  which  can  be  shifted  hori- 
zontally or  radially  on  loosening  one  screw.    The  meter  registers  directly  iq  kUgwatt  houi* 


AMMETERS,    VOLTMETERS,  AND   WATTMETERS        567 


5.  In  making  the  circuit  connections,  be  very  careful  that  the  positive 
lead  or  wire  is  placed  in  the  positive  binding  post  of  the  meter.  This 
precaution  is  essential  for  insuring  an  accurate  and  sensitive  measure- 
ment on  small  loads. 

6  When  a  meter  of  the  commutated  motor  type  sparks  at  the  brushes 
at  starting,  it  is  an  indication  that  the  commutator  is  dusty.  Clean 
it  with  a  piece  of  closely  woven  cotton  tape  ^-inch  in  width. 


Pig.  660. — Interior  of  Thompson  pre- 
payment watt  hour  meter.  The  actu- 
ating force  is  a  large  flat  coil  spring 
enclosed  in  a  barrel  or  drum  to  which 
its  outside  end  is  attached.  The 
operating  knob  winds  this  main  spring 
by  turning  the  drum.  The  spring 
has  many  turns  and  as  the  operation 
of  the  device  never  equals  one  whole 
turn,  the  spring  always  exerts  a 
practically  constant  force.  The  rate 
device  consists  of  a  small  train  of 
gears  secured  to  the  front  of  the 
frame  directly  back  of  the  register. 
Each  device  is  marked  with  the  price 
per  kw-hr.  for  which  it  should  be 
used.  The  switch  is  of  the  double 
pole  double  break  type  with  leaf 
contacts.  The  coin  receptacles  are 
placed  at  the  back  of  the  meter.  To 
make  an  advance  payment,  the  wind- 
ing knob  is  turned  so  that  the  arrow 
points  upward.  A  quarter  dollar  is 
then  inserted  in  the  slot  and  the  knob 
turned  to  the  right,  the  coin  serving 
as  a  key  which  operates  the  mechan- 
ism within  the  device,  turning  the 
registering  wheel  and  placing  the  coin 
to  the  credit  of  the  customer.  If 
the  circuit  be  open  when  the  coin 
is  deposited  the  same  motion  of  the 
knob  which  moves  the  registering 
mechanism  closes  the  circuit  switch 
contained  within  the  case.  The  dial 
contains  a  scale  marked  in  plain 
figures  over  which  a  pointer  passes  indicating  the  number  of  coins  remaining  to  the  credit 
of  the  depositor.  When  the  first  coin  is  deposited  and  the  knob  turned  closing  the  main 
switch,  the  pointer  rests  opposite  the  first  division  on  the  scale.  If  a  second  coin  be  de- 
posited before  the  current  purchased  with  the  first  coin  has  been  consumed,  a  second 
motion  of  the  knob  will  bring  the  pointer  opposite  the  second  division  on  the  scale. 
Twelve  coins  can  thus  be  deposited  consecutively,  after  which  the  slot  is  automatically 
closed  and  further  prepayment  cannot  be  made  until  the  value  of  one  or  more  coins  has 
been  consumed.  Whenever  energy  to  the  value  of  one  coin  has  been  delivered  through 
the  meter,  the  escapement  is  mechanically  released  turning  the  pointer  back  one  division. 
This  process  continues  until  all  the  energy  has  been  delivered  for  which  payment  has 
been  made.  Thus  the  depositor  can  ascertain  at  any  time  how  much  energy  can  be  obtained 
without  further  payment.  When  all  energy  has  been  delivered,  the  circuit  switch  is  opened 
so  that  no  more  current  can  be  obtained  until  one  or  more  coins  have  been  deposited. 
The  indicating  mechanism  shows  only  the  number  of  coins  which  stand  to  the  credit  of 
the  customer,  but,  by  consulting  the  meter  dial,  one  can  determine  what  fractional  part 
of  the  prepayment  next  to  be  cancelled  remains  to  the  credit  of  the  customei.  A  coin 
or  washer  larger  than  the  coin  for  which  the  device  is  designed  cannot  be  introduced  into 
the  receiving  slot  and  a  smaller  one  will  not  operate  the  device. 


568  HAWKINS  ELECTRICITY 


Meters  should  never  be  allowed  to  remain  with  their  covers  off,  in 
the  testing  room,  station,  or  any  other  place.  In  order  to  get  the  best 
service,  and  to  give  them  long  life  they  must  be  kept  clean. 


How  to  test  a  meter. — A  simple  test  for  ascertaining  whether 
a  customer's  meter  is  fast  or  slow,  may  be  applied  as  follows: 

1.  Turn  off  the  lamps  and  other  power  consuming  devices  in  the  house 
and  then  note  the  reading  of  the  meter  dial  and  the  exact  time  of  day; 

2.  Turn  on  as  quickly  as  possible  about  one-tenth  of  all  the  lamps  in 
the  house  and  allow  them  to  burn  for  about  two  hours; 

3.  At  the  end  of  two  hours,  turn  oflf  the  lamps  as  quickly  as  possible 
and  note  the  reading  of  the  meter  dial. 

The  difference  between  the  first  and  second  readings  of  the  dial  will  be 
the  indicated  consumption  of  two  hours,  and  if  this  be  greater  than  the 
amount  of  power  that  ought  to  be  consumed  by  the  number  of  lamps 
turned  on,  the  meter  is  fast,  but  if  it  be  less,  the  meter  is  slow. 

The  best  results  obtained  by  this  method  are  only  approximations, 
however,  on  account  of  the  variations  in  the  watts  consumed  by  the 
different  makes  of  lamp,  the  uncertainty  as  to  the  actual  voltage  on 
the  line  at  the  time  of  the  test,  and  the  lack  of  knowledge  as  to  the  age 
of  the  lamps.  Therefore,  if  the  meter  test  within  five  per  cent.,  or  do 
not  record  more  nor  less  than  one-twentieth  of  the  assumed  lamp  con- 
sumption it  is  safe  to  assume  that  the  meter  is  correct  as  the  result  of 
the  test  is  not  likely  to  be  any  closer  to  the  truth.  ;  ^ 

Oues.     How    should    a    roughened     commutator    be 
cleaned  and  smoothed  ? 

Ans.      By  means  of  tape. 


NOTE. — A  meter  operates  under  more  varied  and  exactinR  conditions  than  almost  any 
other  piece  of  apparatus.  It  is  frequently  subjected  to  ^^bration.  moisture  and  extremes  of 
temperature;  it  must  register  accurately  on  varying  voltages  and  various  wave  forms;  it  must 
operate  for  many  months  without  any  supervision  or  attention  whatever;  and,  in  spite  of  all 
these  conditions,  it  is  expected  to  register  with  accuracy  from  a  few  per  cent,  of  its  rated  capacity 
to  a  50  per  cent,  overload.  As  a  meter  is  a  type  of  machine,  its  natural  tendency  is  to  run  slow; 
but  occasionally,  through  accident,  a  meter  may  run  fast.  When  a  meter  runs  fast  the  con- 
sumer is  paying  a  higher  rate  per  kilowatt  hcur  than  his  contract  calls  for.  He  is  being  dis- 
criminated against.  The  periodic  testing  of  meters  is  therefore  a  necessity  and  is  an  indication 
of  the  honesty  of  intention  of  the  manager  toward  the  customers  of  his  company.  Meters 
controlling  a  very  large  amount  of  revenue  may  be  tested  as  often  as  once  a  month,  while  the 
ordinary  run  of  meters  should  be  tested  at  least  once  a  year,  once  in  eighteen  months,  or  once  in 
two  yeais.  the  period  varying  with  different  companies,  different  types  and  different  civic 
requirements.  Commutator  t>-pe  meters.  ha\'ing  comparatively  hea\-y  moN-ing  elements  with 
consequent  rapid  increase  in  friction  due  to  wear  on  the  jewel  and  bearings,  and  a  commutator 
also  increasing  in  friction  \*ith  age,  must  have  frequent  and  expert  attention  to  insure  their 
accuracy  under  all  conditions. 


AMMETERS,    VOLTMETERS,   AND    WATTMETERS 


569 


.  661. — Internal  connections  of  Sangamo  watt  hour  meter  (type  D).  A,  copper  disc 
armature,  submerged  in  mercury;  B,  bridge  wire  between  binding  posts,  for  main  load 
current,  when  both  sides  of  the  line  are  carried  through  the  meter;  CT,  compounding 
series  turns  around  pressure  circuit  magnet,  building  up  field  as  load  increases,  to  compen- 
sate for  falling  off  in  speed  otherwise  found;  D,  aluminum  damping,  or  brake  disc,  con- 
trolling speed  of  meter;  E,  copper  contact  ears,  imbedded  in  insulating  wall  of  mercury 
chamber,  leading  current  into  and  out  from  armature;  F,  hardwood  float  on  armature 
proportioned  to  give  slight  "lift"  to  entire  moving  system,  when  armature  and  float  are 
immersed  in  mercury;  H,  soft  steel  disc  above  permanent  magnets,  riveted  to  fine  pitch 
screw  working  in  bracket  above,  so  that  adjustment  of  the  disc  up  or  down  gives  variation 
in  damping  effect  of  permanent  magnets,  and  therefore  of  main  speed.  K,  clamp  slider 
with  thumb  screw,  for  obtaining  light  load  adjustment  by  mo\'ing  K  to  right  or  left,  as 
may  be  necessary.  K  spans  and  connects  parallel  wires  of  light  load  adjustment,  BR 
and  RR'.  MM,  powerful  permanent  magnets,  acting  on  disc  D,  giving  main  speed  con- 
trol for  meter.  N,  high  resistance  heav^  wire,  forming  part  of  series  adjustment  between 
armature  and  any  shunt  with  which  meter  may  be  used,  to  set  drop  through  meter  correct 
for  drop  of  the  shunt.  P,  spirally  laminated  soft  steel  ring,  moulded  in  mercury  chamber 
above  the  armature  space,  to  act  as  a  return  for  magnetic  lines  of  force  from  and  to  ener- 
gizing magnet  below.  R,  resistance  card  unit,  in  series  with  pressure  circuit  coils;  in 
110  volt  meters  one  card  is  used,  in  220  volt  meters  two  cards,  or  one  card  and  a  thermo- 
couple. BR,  small  brass  wire,  connected  to  ingoing  end  of  pressure  circuit  coils  and  form- 
ing RR'and  the  slides  K  the  light  load  adjustment.  RR',  high  resistance  wire  having 
opposite  ends  connected  to  ears  EE  by  low  resistance  wires.  Current  energizing  the 
pressure  circuit  coils  SC  passes  from  RR'  through  K  to  BR  and  thence  to  the  coils,  and 
if  K  be  near  the  end  of  RR'  and  BR,  the  least  compensation  is  obtained;  if  near  right  end, 
maximum  light  load  compensation  is  obtained.  S,  shaft  or  -spindle.  In  actual  meter  S 
is  divided,  the  lower  shaft  carrying  armature  A,  and  the  upper  shaft  damping  disc  D. 
SA,  series  resistance  adjustment,  for  setting  meter  to  correct  drop  for  shunt.  SC  and 
SC,  pressure  coils  connected  in  serie«.  TT,  binding  posts  at  bottom  of  meter.  Y,  lami- 
nated soft  steel  yoke,  carrying  coils  SC  and  SC,  and  giving  a  powerful  and  concentrated 
magnetic  field  on  the  armature.   W,  worm,  driving  recording  dial  train.  WW,  worm  wheel. 


570 


HAWKINS  ELECTRICITY 


Waste  of  Electricity  in  Lighting. — In  large  residences  where 
a  good  many  servants  are  employed  or  in  any  plaee  where  the 
power  consumed  is  not  directly  under  the  supervision  of  the  per- 
son who  must  pay  the  bills,  a  great  deal  of  waste  usually  occurs. 


CYLINDRICM 

(BRUSHES. 

COMMUTATOR 

DIAMETER 

ONE-TENTH, 

INGHx 


ARMATURE  COSISTS 
OF  THREE  INTER 
LOCKED  COILS. 


DlREa  READING 

ASSOCIATION  :;„ 

TRAIN  aV  DRIVEN 


MICROMETER  SCREW  ADJUSTER 
"■"  REGULATING  BRUSH  TENSION. 

BRUSH  CLAMP  ENABLING 
SWINGING  BRUSHES 
CLEAR  OF  COMMUTATOR 
AND  REENGAGING  WITH- 
OUT ALTERING  TENSION. 

RIGID  ONE  PIECE  CAST 
^  ALUMINUM  BASE  WITH 
i  STIFFENER  RIBS- 

SHORTSTIFF  SHAFT 
WITH  PIVOTS  FRiaiON 
CLAMP  ATTACHED 


LIGHT  LOAD  ADJUST- 
MENT OF  WIDE  RANGE. 


DAMPER  MAGNETS 

-IMMOVABLY  CLAMPED 

IN  PLACE. 


MICROMETER  MAGNETIC 
SHUNT  ADJUSTMENT  FOR 

CONTR0LUN6  MAIN  SPEED  ALUMINUM     JVNEL 

BEARING. 

Pig.  662. — Interior  view  of  Columbia  watt  hour  meter  (type  T)),  showing  construction  and 
principal  parts  and  connections.  The  armature  winding  consists  of  three  coils  appro.^i- 
mately  circular  in  shape.  The  coils  are  for.m  wound,  interlocked  with  one  another  and  with 
the  light  impregnated  fibre  disc  which  serves  as  a  spacer  for  them.  The  aluminum  damper 
disc  has  the  conventional  anti-creep  provision  in  the  shape  of  the  three  small  soft  iron 
plugs,  mounted  close  to  the  central  staff,  which  the  illustration  shows.  These  in  their 
revolution  come  successively  within  the  influence  of  an  adjustable  iron  screw  which  is 
magnetized  by  an  extension  from  one  of  the  damper  magnets.  The  angular  relationship 
of  the  armature  windings  and  of  the  three  iron  plutrs  is  such  that  at  the  time  that  the 
armature  is  exerting  a  maximum  torque  the  magnetized  screw  is  exerting  the  maximum 
pull  to  hold  back  a  given  plug  and  conversely  when  the  armature  pull  is  a  minimum 
the  magnetic  screw  is  attracting  a  plug  with  the  maximum  effort  to  cause  ahead  rotation. 
The  irregularities  of  torque  are  in  this  way  smoothed  out.  The  commutator  has  three 
segments  and  is  made  of  chemically  pure  silver.  Each  brush  is  formed  of  a  length  of 
phosphor  bronze  wire  bent  like  a  hair  pin  and  secured  at  its  "U"  end  to  a  brass  sleeve, 
which  in  turn  is  secured  to  an  insulated  stud  by  a  set  screw.  An  extension  on  the  sleeve 
carries  a  micrometer  screw  brush  adjustment.  The  main  speed  adjustment  is  secured  by 
pro\Hding  a  soft  iron  bridge  plate,  bridging  over  the  extremities  of  each  magnet  end  and 
adjustable,  by  means  of  a  set  screw  and  lock  nut.  to  any  desired  distance  therefrom.  This 
gives  a  regular  micrometer  means  of  varying  the  effective  magnet  strength.  Interposed 
between  the  series  coil  and  the  permanent  magnets  is  a  heavy  soft  iron  shield  to  guard  the 
magnets  against  disturbance  by  short  circuiting.  Light  load  adjustment  is  obtained  by 
pro\nding  in  the  coil  circuit  a  series  of  small  resistance  spools,  equipped  with  pin  terminals, 
to  which  connection  can  be  selectively  made  by  means  of  a  split  bushing  terminal  on  a 
flgadble  cord.    This  series  of  spools  is  stniog  on  a  metal  artx>r  located  withiQ  uie  case. 


AMMETERS,    VOLTMETERS,  AND   WATTMETERS  571 


TTRKUNALS 


TERMINALS 


C0MPEW5MIN& 
COIL  SWITCH 


Fig.  663. — Diagram  showing  internal  connections  of  the  Duncan  watt  hour  meter.  Its  opera- 
tion depends  upon  the  principle  of  the  well  known  electro-dynamometer,  in  which  the 
electromagnetic  action  between  the  currents  in  the  field  coils  and  an  armature  pro- 
duces motion  in  the  latter.  It  also  embodies  the  other  two  necessary  watt  hour  meter 
elements  required  for  the  speed  control  and  registration  of  the  revolutions  of  the  armature, 
these  being  embodied  in  the  drag  magnet  and  disc,  and  the  meter  register  respectively, 
The  motion  of  the  armature  is  converted  into  continuous  rotation  by  the  aid  of  a  com» 
mutator  and  brushes,  the  commutator  being  connected  to  the  armature  coils  and  carried 
on  the  same  spindle  therewith. 


572  HAWKINS  ELECTRICITY 


If  the  meter  be  read  before  retiring,  the  reading  in  the  morning  will 
show  how  much  energy  was  consumed  during  the  night,  which  will 
show  in  turn  how  many  lamps  were  burning  all  night. 

A  great  deal  of  light  can  be  saved  by  placing  the  lamps  so  that  they 
will  throw  the  light  where  it  is  needed  and  by  placing  small  lamps  such 
as  8  candle  power  and  4  candle  power  in  places  where  not  much  light  is 
needed,  such  as  bathrooms,  halls,  cellars,  etc. 

When  the  lamps  get  old  and  dim  they  should  be  replaced  with  new 
ones,  as  it  costs  about  the  same  to  bum  an  old  lamp  as  a  new  one.  An 
old  16  candle  power  lamp  which  is  very  dim  will  give  only  about  8 
candle  power  and  use  about  as  much  current  as  is  required  for  a  new 
16  candle  power.  If  the  dim  light  be  light  enough,  it  should  be  replaced 
by  an  8  candle  power  lamp,  which  will  not  consume  as  much  power 
as  the  old  16  candle  power. 


OPERA  TION  OF  D  YNA  MOS  573 


CHAPTER  XXIX 
OPERATION  OF  DYNAMOS 


Before  Starting  a  Dynamo  or  Motor. — When  the  machine 
has  been  securely  fixed,  it  should  be  carefully  examined  to  see 
that  all  parts  are  in  good  order.  The  examination  should  be 
made  as  follows : 

1.  The  field  magnet  circuit  should  first  be  inspected  to  see  that 
none  of  the  wires  or  connections  have  broken  or  have  become 
loose,  and  that  the  coils  are  correctly  connected; 

2.  The  caps  of  the  bearings  should  be  taken  off,  and  these 
and  the  journals  carefully  cleaned  of  all  grit  and  dirt.  They 
should  then  be  oiled,  and  the  caps  replaced  and  screwed  up 
by  hand  only; 

3.  The  gaps  between  the  outer  surface  of  the  armature  and 
the  polar  faces  should  be  examined  in  order  to  ascertain  whether 
any  foreign  body,  such  as  a  small  screw  or  nail  has  lodged  therein. 
If  such  be  the  case,  it  should  be  carefully  removed  with  a  bit  of 
wire; 

4.  The  guard  plates  protecting  the  armature  windings  should 
be  removed,  and  the  windings  carefully  inspected  by  slowly 
rotating  the  armature,  to  see  that  they  are  not  damaged,  and 
that  the  insulation  is  perfect.  The  armature  should  then  be 
finally  rotated  by  hand  to  see  that  it  revolves  freely,  and  that 
the  bearings  are  securely  fixed; 


574  HAWKINS  ELECTRICITY 

5.  The  commutator  should  be  examined  to  see  that  it  is  not 
damaged  in  any  way  through  one  or  more  of  the  segments  being 
knocked  in,  or  the  lugs  being  forced  into  contact  with  one 
another ; 

6.  The  brush  holders  and  brushes  should  be  inspected  to  see 
that  the  former  work  freely  on  the  spindle,  and  that  the  hold  off 
catches  work  properly,  are  clean  and  make  good  contact  with 
the  brush  holders  or  flexible  leads ; 

7.  Ha^•ing  ascertained  that  the  machine  is  not  injxired  in  any 
way,  and  that  the  armattu-e  revolves  freely,  the  brushes  should 
be  adjusted. 

In  the  subsequent  working  of  the  d>Tiamo  it  •will  of  course  be  un- 
necessan,-  to  follow  the  whole  of  these  proceedings  ever\'  time  the  machine 
is  started,  as  it  is  extremely  unlikeh'  that  the  machine  will  be  damaged 
from  external  causes  while  working  without  the  attendant  being  aware  of 
the  fact. 

Adjusting   the    Brushes. — The   adjustment   of  the  brushes 

upon  the  commutator  requires  careful  attention  if  sparking  is 
to  be  avoided.    There  are  two  adjustments  to  be  made: 

1.  For  pressure; 

The  brushes  naust  bear  against  the  commutator  segments  with 
sufficient  pressure  for  proper  contact. 

2.  For  lead. 

The  brushes  must  have  the  proper  angular  advance  (positive  or 
n^ative,  according  as  the  machine  is  a  d\-namo  or  motor)  to  prevent 
sparking. 

Oues.  At  what  point  on  the  commutator  should  the 
brushes  bear? 

Ans.  The  points  upon  the  commutator  at  which  the  tips  of 
the  brushes  (carried  by  opposite  arms  of  the  rocker)  bear,  should 


OPERATION  OF  DYNAMOS 


575 


be,  in  bipolar  dynamos,  at  opposite  extremities  of  a  diameter. 
In  multipolar  dynamos  the  positions  vary  with  the  number  of 
poles  and  the  nature  of  the  armature  winding. 

Ones.    What  provision  is  made  to  facilitate  the  correct 
setting  of  the  brushes? 

Ans.     Setting  marks  are   usually  cut   in  the   collar   of   the 
commutator  next  to  the  bearing. 


Figs.  664  and  665. — Diagrams  illustrating  how  to  set  brushes.  Some  brush  holders  require  brushes 
set  with  the  direction  of  rotation  of  the  commutator,  and  others,  set  against  the  direction 
of  rotation.  In  fig.  0G4  is  shown  a  brush  holder  of  the  first  class,  which  must  always  be  set 
as  indicated  by  the  arrow.  If  set  in  the  opposite  direction,  trouble  will  ensue,  as  an 
inspection  of  the  figure  will  show,  because  the  surface  of  the  commutator  and  the  brush 
would  form  a  toggle  joint,  and  the  brush  would  tend  to  dig  into  the  commutator  and 
either  break  itself  or  bend  the  brush  rigging.  In  fig.  6G5  is  shown  a  brush  holder  of  the 
second  type.  This  brush  is  set  against  the  direction  of  rotation,  but  an  inspection  of  the 
cut  will  show  that  there  is,  in  this  case,  no  tendency  for  the  brush  to  dig  into  the  com- 
mutator surface.  Each  type  of  brush  holder,  of  which  there  are  several,  should  be  adjusted 
as  recommended  by  the  manufacturer  to  secure  proper  working. 


Ques.     How  are  the  brushes  set  by  these  marks? 

Ans.  The  tips  of  all  the  brushes  carried  by  one  arm  of  the 
rocker  are  set  in  correct  line  with  the  commutator  segments 
marked  out  by  one  setting  mark,  and  the  tips  of  the  brushes 
carried  by  the  other  arm  or  arms  are  set  in  correct  line  with  the 
segments  marked  out  by  the  other  mark  or  marks. 

If  one  or  more  of  the  brushes  in  a  set  be  out  of  line  with  their  setting 
mark,  it  will  be  necessary  to  adjust  the  brushes  up  to  this  mark  by  push- 
ing them  out  or  drawing  them  back,  as  may  be  required,  afterwards 


67G 


II A  WKINS  ELECTRICITY 


clamping  them  in  position.  When  adjusting  the  brushes,  the  armature 
should  always  be  rotated,  so  that  the  setting  marks  are  horizontal.  The 
rocker  can  then  be  rotated  into  position,  and  the  tips  of  both  sets  of 
brushes  conveniently  adjusted  to  their  marks.  In  those  brush  holders 
provided  with  an  index  or  pointer  for  adjusting  the  brushes,  the  setting 
marks  upon  the  commutator  are  absent,  length  of  the  pointer  being  so 
proportioned  that  when  the  tips  of  the  brushes  are  in  line  with  the  ex- 
treme tips  of  the  pointers,  the  brushes  bear  upon  the  correct  positions  on 
the  commutator. 


Fig.  666. — Method  of  soldering  cable  to  carbon  brush.  Drill  a  hole  in  the  end,  also  in  the  side 
of  the  brush,  as  shown  in  the  sketch,  and  after  thoroughly  tinning  the  "pig-tail,"  place 
it  in  the  end  hole  and  fill  the  holes  up  with  solder  through  the  side  hole.  Another  method 
is  to  drill  a  hole  through  the  carbon  so  that  the  cable  will  just  slip  through,  countersink 
the  edge  of  the  hole  a  little,  clean  the  cable  thoroughly  and  pass  it  through  the  hole. 
Then  with  any  good  flu.x  and  solder,  fill  the  countersunk  part  on  both  sides. 


Oues.  What  should  be  done  after  adjusting  the  brushes 
to  their  correct  positions  upon  the  commutator  ? 

Ans.  Their  tips  or  rubbing  ends  should  be  examined  while 
in  position  to  see  that  they  bed  accurately  on  the  surface  of 
the  commutator. 

In  many  instances  it  will  be  found  that  this  is  not  the  case,  the 
brushes  sometimes  bearing  upon  the  i)oint  or  toe,  and  sometimes  upon 
the  heel,  so  that  they  do  not  make  contact  with  the  commutator  through- 
out their  entire  thickness  and  width.  The  angle  of  the  rubbing  ends 
will  therefore  need  to  be  altered  by  filing  to  make  them  lie  flat. 


OPERA  TION  OF  D  YNA  MOS  bll 

Ques.     How  is  the  proper  brush  contact  secured  ? 

Ans.  When  the  brushes  do  not  bed  properly  they  should  be 
refitted  to  secure  proper  contact. 

Ques.     How  is  the  pressure  adjustment  made? 

Ans.  This  is  effected  by  regulating  the  tension  of  the  springs 
provided  for  the  purpose  upon  the  brush  holders. 

Ques.  With  what  pressure  should  the  brushes  bear 
against  the  commutator? 

Ans.  The  tension  of  the  springs  should  be  just  suiKcient  to 
cause  the  brushes  to  make  a  light  yet  reliable  contact  with  the 
commutator. 

The  contact  must  not  be  too  light,  otherwise  the  brushes  will  vibrate, 
and  thus  cause  sparking;  nor  must  it  be  too  heavy,  or  they  will  press  too 
hard  upon  the  commutator,  grinding,  scoring  and  wearing  away  the 
latter  and  themselves  to  an  undesirable  extent,  and  moreover,  giving 
rise  to  heating  and  sparking. 

The  correct  pressure  is  attained  when  the  brushes  collect  the  full 
current  without  sparking,  v/hile  their  pressure  upon  the  commutator 
is  just  sufficient  to  overcome  ordinary  vibration  due  to  the  rotation  of 
the  commutator. 


Direction  of  Rotation. — This  is  sometimes  a  matter  of 
doubt  and  often  results  in  considerable  trouble.  As  a  general 
rule,  a  dynamo  is  intended  to  run  in  a  certain  direction;  either 
right  handed  or  left  handed  according  to  whether  the  armature, 
when  looked  at  from  the  pulley  end,  revolves  with  or  against 
the  direction  of  the  hands  of  a  clock.  Dynamos  are  usually 
designed  to  run  right  handed,  but  the  manufacturers  will  make 
them  left  handed  if  so  desired. 

It  may  be  necessary  to  reverse  the  direction  of  rotation  of  a 

■  dynamo,  if  the  driving  pulley  to  which  it  has  to  be  connected 

happen  to  revolve  left  handed,  or  if  it  be  necessary  to  bring  the 

loose  side  of  the  belt  on  top  of  the  pulley,  or  to  place  the  machine 


>78 


HAWKIXS  ELECTRICITY 


Figs.  667  to  669. — Method  of  winding  cables  with  marlin.  When  connecting  the  feeders  and 
djTiamo  and  se^^^ce  leads  to  a  switchboard,  the  wires  are  often  served  with  marlin.  By 
serving  is  meant  to  tightly  wrap  the  wires  of  each  set  together  with  marlin.  A  tool  for 
ser\-ing  may  be  made  as  in  fig.  667,  using  a  piece  of  oak  2  ins.  wide,  J  g  in.  thick  and  14  ins. 
long,  ha%-ing  four  holes  drilled  through  it,  as  shown.  The  marlin  is  passed  through  the 
holes,  commencing  at  the  hole  nearest  the  handle,  the  object  being  to  cause  a  strain  on 
the  marlin  at  the  point  where  it  passes  around  the  wire,  so  that  the  marUn  may  be  wrapped 
tightly.  It  is  necessary  to  serve  the  first  four  or  five  inches  by  hand,  pushing  the  winding 
into  the  conduit  as  far  as  possible.  This  acts  as  an  additional  protection  to  the  wres 
where  they  leave  the  conduit.  The  serving  is  contir.ued,  as  in  fig.  668,  to  within  four  or 
five  inches  of  the  first  lug  by  means  of  the  serv-ir.g  tool,  passing  the  ball  of  marlin  around 
the  wires  with  the  serving  tool.  The  wires  are  then  bent  in  shape,  as  in  fig.  66;t.  To  serve 
the  wires  properly  it  is  necessar>'  to  tie  the  ends  of  the  wires  taut.  The  wires  should  be 
straightened  and  run  together  so  as  to  be  parallel,  being  bound  with  tape  at  different 
points  to  keep  them  so.  When  the  serving  is  complete  the  marlin  should  be  thoroughly 
painted  with  a  moisture  resisting  compound.  The  marlin  ser\-ing  will  stiffen  the  wires 
and  they  can  be  bent  very  neatly  to  avoid  touching  the  bus  bars  of  the  board.  When 
painted  the  marlin  hardens  so  that  it  is  difficult  to  bend  the  wires  after  the  paint  has  dried. 
It  then  requires  a  strong  pressure  to  bend  them.  The  marlin  acts  as  an  additional  insula- 
tion and  mechanical  protection  to  the  wires,  and  while  no  harm  would  result  from  the 
wires  coming  in  contact  with  the  bars  while  thus  protected,  it  looks  better  to  bend  them  so 
as  to  avoid  touching  the  bars.  />Hpf»^T"y  ^ 


l.li.COllEGfl^^»^V 


OPERA  TION  OF  D  YNA  MOS 


579 


in  a  certain  position  on  account  of  limited  space.  The  direction 
of  rotation  of  ordinary  series,  shunt,  or  compound  bipolar 
dynamos  may  be  reversed  by  simply  reversing  the  brushes 
without  changing  any  of  the  connections,  then  changing  the 
point  of  contact  of  the  brush  tips  180°. 

In  multipolar  dynamos,  a  similar  change,  amounting  to  90° 
for  a  four  pole  machine,  and  45°  for  an  eight  pole  machine,  will 
reverse  their  direction  of  rotation.  It  will  be  understood  that 
under  these  conditions,  the  original  direction  of  the  current  and 
the  polarity  of  the  field  magnets  will  remain  unchanged. 


Fig.  670. — Method  of  assembling  core  discs.  For  this  operation  two  wooden  "horses  "  'should  be 
provided  to  support  the  core  at  a  convenient  height,  as  shown  in  the  illustration. 


This  rule  does  not  apply  to  arc  dynamos  and  other  machines, 
which  have  to  be  run  in  a  certain  direction  only,  in  order  to  suit 
their  regulating  devices. 

If  the  direction  of  current  generated  by  a  dynamo  be  opposite 
to  that  desired,  the  two  leads  should  be  reversed  in  the  ter- 
minals, or  the  residual  magnetism  should  be  reversed  by  a 
current  from  an  outside  sovu-ce^  JNn^^y^^y.  «^ 

*'^if  Cr^fr CAT »,«•*.«.. 


580 


HAWKINS  ELECTRICITY 


Starting  a  Dynamo. — Ha\-ing  followed  the  foregoing  in- 
structions, all  keys,  spanners,  bolts,  etc.,  should  be  removed 
from  the  immediate  neighborhood  of  the  machine,  and  the 
d\Tiamo  started. 


Figs.  671  and  672. — Tinning  block  for  electric  soldering  tooL  It  is  made  with  two  soft  bricks. 
One  brick  is  used  to  support  the  soldering  tool,  and  the  other  to  contain  the  tinning 
material  and  to  furnish  a  material  which  will  keep  the  copper  bit  bright  enough  to  receive 
its  coating  of  "tin."  Fig.  671  represents  a  section  of  the  tinning  brick,  which  is  scooped 
out  on  top  as  shown  by  the  lower  line.  Into  one  end  of  the  hollow  in  the  brick,  some 
sal-ammoniac  is  placed  to  help  tin  the  copper  bit.  Sal-ammoniac  is  a  natural  flux  for 
copper  and  aids  greatly  in  keeping  the  tool  well  tinned.  Next,  some  melted  solder  is  run 
into  the  hollow  of  the  brick,  and  lastly  enough  resin  to  fill  the  ca\-ity  nearly  to  the  top. 
When  the  tool  is  not  in  use,  the  electricity  is  switched  off  and  the  tool  permitted  to  lie  in 
the  resin.  If  it  be  desired  to  repair  the  tin  coating  a  little  when  the  tool  is  in  use,  the 
latter  is  rubbed  on  the  brick  below  the  layer  of  solder,  and  the  layer  of  resin.  If  the  tool 
be  in  ver>'  bad  condition,  it  may  be  pudied  into  the  sal-ammoniac  once  or  twice  and  then 
rubbed  in  the  solder  again.  It  requires  but  little  heat  to  keep  the  brick  and  its  contents 
ready  for  use.  In  fact,  the  brick  is  a  fair  non-conductor  of  heat  and  prevents  the  escap>e 
of  heat  from  one  side  of  the  tool.  When  momentarily  not  in  use,  the  tool  remains  in  the 
solder  which  becomes  melted  underneath  the  layer  of  resin.  When  the  copper  bit 
becomes  too  hot,  it  will  begin  to  volatilize  the  resin,  thus  f-alling  attention  to  this  fact, 
■mbereapoa,  the  electhdty  should  be  tumed  oS  from  the  tooL 


OPERA  TION  OF  D  YNA  MOS 


581 


Oues.    How  should  a  dynamo  be  started? 

Ans.  A  dynamo  is  usually  brought  up  to  speed  either  by 
starting  the  driving  engine,  or  by  connecting  the  dynamo  to  a 
source  of  power  already  in  motion.  In  the  first  case,  it  should 
be  done  by  a  competent  engineer,  and  in  the  second  case  by  a 
person  experienced  in  putting  on  friction  clutches  to  revolving 
shafts,  or  in  slipping  on  belting  to  moving  pulleys. 


l-'iG.  673. — Connections  for  two  shunt  wound  dynamos  to  run  in  parallel.  The  positive  lead  of 
each  machine  is  connected  to  the  same  bus  bar.  In  starting,  if  but  one  machine  is  to  be 
used,  the  dynamo  is  first  brought  up  to  speed  and  the  voltage  regulated  by  means  of  the 
rheostat  R  and  the  voltmeter  V.  The  main  switch  is  then  thrown  in.  The  connections 
for  the  field  are  taken  off  the  dynamo  leads  so  that  the  opening  of  the  main  switch  will  not 
open  the  field  circuit  and  for  this  reason  the  field  will  begjin  to  build  up  as  soon  as  the 
machine  is  started.  When  but  one  of  the  machines  is  running,  the  idle  machine  is  brought 
up  to  speed  with  the  main  switch  open,  and  the  voltage  regulated  by  means  of  the  rheostat 
and  voltmeter  until  the  voltages  of  the  machines  are  the  same.  Then  the  main  switch  is 
thrown  in  and  the  load  on  the  machines  (which  is  ascertained  by  the  ammeters)  is  equalized 
by  means  of  the  rheostats.  Should  there  be  any  great  difference  in  voltages,  the  higher  one 
will  run  the  other  as  a  motor  without  changing  the  direction  of  rotation.  The  field  current 
will  remain  unchanged,  and  the  armature  current  of  the  low  dynamo  will  be  reversed, 
which  will  cause  it  to  run  as  a  motor  in  the  same  direction  as  it  ran  as  a  dynamo.  When 
dynamos  feeding  current  to  motors  are  to  be  shut  down,  the  switches  on  the  motors  should 
first  be  opened.  Otherwise  some  of  the  motor  fuses  will  blow.  As  the  voltage  goes  down 
the  motors  will  draw  more  current  to  do  the  work.  If  a  plant  be  shut  down  with  the 
motor  switches  "in"  it  will  generally  be  found  impossible  to  start  a  shunt  dynamo,  the 
low  resistance  in  the  mains  not  allowing  enough  current  to  flow  around  the  shunt  fields 
to  energize  them. 


582 


HAWKINS  ELECTRICITY 


Ques.  Should  the  brushes  be  raised  out  of  contact  in 
starting? 

Ans.  The  brushes  should  not  be  in  contact  in  starting  if 
there  be  any  danger  of  reverse  rotation,  as  might  happen  when 
the  dynamo  is  driven  by  a  gas  engine.  Aside  from  this,  it  is 
desirable  that  the  brushes  be  in  contact,  because  they  are  more 

NEGATIVE     BUS 


SMUMT    DYNAMO  SHUMT    DVKAMO 

Fig.  674. — Connections  for  two  shunt  dynamos  to  run  on  the  three  «^Tre  sj-stem.  The 
two  machines  are  connected  in  series,  three  wires  being  carried  from  them,  one  from  the 
outside  pole  of  each  machine  and  one  from  the  .iunciion  of  the  two  machines.  The  voltage 
between  the  outside  wires  is  equal  to  the  combined  voltage  of  the  two  machines  and  the 
voltage  between  the  outside  and  the  central  or  neutral  wire  is  equal  to  the  voltage  of 
the  corresponding  machine.  If  the  load  on  each  side  of  the  system  be  equal,  there  will  be 
no  current  in  the  neutral  wire,  while  if  the  loads  be  unequal,  the  neutral  wire  will  have  to 
carrv'  the  difference  in  current  between  the  two  outside  wires. 


easily  and  better  adjusted,  and  the  voltage  will  come  up  slowly, 
so  that  any  fault  or  difficulty  will  develop  gradually  and  can  be 
corrected,  or  the  machine  stopped  before  any  injury  is  done. 

Ques.     How  should  a  series  machine  be  started  ? 

Ans.     The   external   circuit   should    be   closed,    otherwise   a 


OPERA  TION  OF  D  YNA  MOS 


583 


closed  circuit  will  not  be  formed  through  the  field  magnet  wind- 
ing and  the  machine  will  not  build  up. 

Oues.    What  is  understood  by  the  term  "  build  up"  ? 

Ans.     In  starting,  the  gradual  voltage  increase  to  maximum. 

A )  ( V )  (T 


Fig.  675. — Connections  for  two  compound  wound  dynamos  to  run  in  parallel.  The  series 
fields  of  the  machines  are  connected  together  in  parallel  by  means  of  wire  leads  or  bus  bars, 
which  connect  together  the  brushes  from  which  the  series  fields  are  taken.  This  is  known 
as  the  equalizer  and  is  shown  by  the  line  running  to  the  middle  pole  of  the  dynamo  switch. 
By  tracing  out  the  series  circuits  it  will  be  seen  that  current  from  the  upper  brush  of  either 
dynamo  has  two  connections  to  its  bus  bar.  One  of  these  leads  through  its  own  field,  and 
the  other,  by  means  of  the  equalizer  bar,  through  the  fields  of  the  other  dynamo.  As  long 
as  both  machines  are  generating  equally  there  is  no  difference  of  pressure  between  the 
brushes  of  either,  but  should  the  voltage  of  one  be  lowered,  current  from  the  other  would 
flow  through  its  fields  and  thereby  raise  the  voltage,  and  at  the  same  time  reduce  its 
own  until  both  are  equal.  The  equalizer  may  then  be  called  upon  to  carry  much  current, 
but  to  have  the  machines  regulate  closely  it  should  be  of  low  resistance.  It  may  also  be 
run  as  shown  by  the  dotted  lines,  but  this  will  leave  all  the  machines  alive  when  any  one 
is  generating.  The  ammeters  should  be  connected  as  shown.  If  they  were  on  the  other 
side  they  would  come  under  the  influence  of  the  equalizing  current  and  would  indicate 
wrong,  either  too  high  or  too  low.  The  equalizer  switch  should  be  closed  a  little  before 
the  main  switches  are  closed. 

Oues.     How  should  a  shunt  or  compound  machine  be 
started  ? 

Ans.     All  switches  controlling  the  external  circuits  should  be 
opened,  as  the  machine  excites  best  when  this  is  the  case.    If 


584  HAWKINS  ELECTRICITY 

the  machine  be  pro\-ided  with  a  rheostat  or  hand  regulator  and 
resistance  coils,  these  latter  shotdd  all  be  cut  out  of  circuit,  or 
short  circuited,  until  the  machine  excites,  when  they  can  be 
gradually  cut  in  as  the  voltage  rises. 

When  the  machine  is  giving  the  correct  voltage,  as  indicated  by  the 
voltmeter  or  pilot  lamp,  the  machine  may  be  switched  into  connection 
with  the  external  or  working  circuits. 

Oues.  In  starting  a  shunt  dynamo,  should  the  main 
line  switch  be  closed  before  the  machine  is  up  to  voltage 
or  after? 

Ans.  If  the  machine  be  working  on  the  same  circuit  with 
other  machines,  or  with  a  storage  batter\',  it  is,  or  course,  neces- 
sary to  make  the  voltage  of  the  machine  equal  to  that  on  the 
line  before  connecting  it  in  the  circuit.  If  the  machine  work 
alone,  the  switch  may  be  closed  either  before  or  after  the  voltage 
comes  up.  The  load  ^\*ill  be  thrown  on  suddenly  if  the  s\^'itch 
be  closed  after  the  machine  has  built  up  its  voltage,  thus  causing 
a  strain  on  the  belt,  and  possibly  dra-^-ing  water  over  the  engine 
cylinder.  On  the  other  hand,  if  the  switch  be  closed  before  the 
voltage  of  the  machine  has  come  up,  the  load  is  picked  up  grad- 
ually, but  the  machine  may  be  slow  or  may  even  refuse  to  pick 
up  at  all. 

Oues.  Why  does  a  shunt  machine  pick  up  more  slowly 
if  the  main  switch  be  closed  first? 

Ans.  Because  the  resistance  of  the  main  line  is  so  much  less 
than  that  of  the  field  that  the  small  initial  voltage  due  to  the 
residual  magnetism  causes  a  much  larger  current  in  the  armature 
than  in  the  shunt  field.  If  this  be  too  large,  the  cross  and  back 
magnetizing  force  of  the  armature  weakens  the  field  more  than 
the  initial  field  cturent  strengthens  it,  and  so  the  machine 
cannot  bxiild  up. 


OPERATION  OF  DYNAMOS 


585 


Ones.  If  a  shunt  dynamo  will  not  pick  up,  what  is  likely 
to  be  the  trouble? 

Ans.  The  speed  may  be  too  slow;  the  resistance  of  the 
external  circuit  may  be  too  small;  the  brushes  may  not  be 
in  proper  position  ;  some  of  the  electrical  connections  in  the 
dynamo  may  be  loose,  broken  or  improperly  made;  the  field 
may  have  lost  its  residual  magnetism. 


© 


FTaIIx 


n 


Pigs.  fi76  and  677. — Diagrams  of  ground  detectors.  Fig.  676,  a  ground  detector  switch  suitable 
for  mounting  on  a  switch  board.  The  two  arms  pivoted  at  their  upper  ends  are  connected 
with  an  insulating  bar  A  and  make  contact  at  their  lower  ends  with  two  brass  strips 
and  a  contact  button,  which  are  connected  to  the  bus  bars  and  ground,  respectively. 
When  the  arms  are  moved  to  the  left,  the  positive  bus  bar  is  connected  to  the  ground 
through  the  voltmeter  V.  In  fig.  677  is  another  form  of  ground  detector.  This  is  known 
as  a  lamp  ground  detector.  On  a  110  volt  system  two  ordmary  lamps  are  connected  in 
series,  while  the  line  connecting  the  lamps  is  connected  to  the  ground  through  a  snap 
switch  vS.  When  current  is  on,  the  two  lamps  will  bum  with  equal  brilliancy,  but  at  a 
lower  candle  power.  When  the  switch  S  is  closed,  if  the  two  lines  be  clear,  the  brilliancy 
of  the  lamps  will  not  be  affected,  but  if  there  be  a  ground  on  the  positive  side,  one  lamp 
will  bum  brighter,  the  brightness  depending  on  the  resistance  of  the  ground.  If  there 
be  a  dead  ground,  the  lamp  will  bum  to  its  full  candle  power. 


Oues.  What  is  the  indication  that  the  connections 
between  the  field  coils  and  armature  are  reversed  ? 

Ans.  If  the  machine  build  up  when  brought  to  full  speed, 
the  connections  are  correct,  but  if  it  fail  to  build  up,  the  field 
coils  may  be  improperly  connected. 


586 


HAWKINS  ELECTRICITY 


POSITIVE  BU5-BA.R 

-♦- 


FIELD 
RHEOSTM 


ARMATURE 


Pig.  678. — Method  of  correcting  reversed  polarity  in  large  shunt  d>Tiaiiio  by  transposing  the 
shunt  field  leads,  and  then  starting  up  the  machine.  As  soon  as  the  voltmeter  registers 
any  voltage,  the  dynamo  may  be  stopped  and  the  field  leads  restored  to  their  original 
position,  when  it  will  be  found  that  the  residual  magnetism  in  the  pole  pieces  will  usually 
bring  the  dynamo  up  to  its  jwlarity  and  proper  voltage.  This  method  has  the  disadvan- 
tages, of  the  uncertainty  as  to  the  machine  building  up,  and  that  a  temporar>'  wire  must 
probably  be  run  from  the  switchboard  to  one  terminal  of  the  field  circuit,  which  is  usually 
connected  to  a  terminal  back  of  the  dynamo  frame,  so  that  the  flow  of  current  through 
the  field  coils  may  be  reversed.  With  d>-namos  ha\-ing  laminated  field  magnet  cores  of 
comparatively  low  residual  magnetism,  this  method  may  suflfice,  but  in  the  case  of  solid 
field  magnetic  cores  it  is  not  practical.  A  better  method  is  to  disconnect  the  shunt 
field  leads  and  temjjorarily  extend  them  to  some  other  source  of  direct  current.  If  the 
current  be  of  higher  voltage  than  the  coils  are  designed  for,  as  for  instance  1 10  volt 
d>T!amo  and  available  current  500  volt,  caution  must  be  exercised  and  a  suitable  resist- 
ance be  pro\-ided  to  protect  the  coils.  A  500  volt  coil,  however,  may  be  supplied  from 
110  volt  circuit,  pro%"iding  the  field  winding  to  be  energized  is  equipp>ed  with  a  cut  off 
switch  ha\-ing  a  discharge  resistance,  so  that  it  may  be  used  to  close  and  break  the  circuit 
when  the  temporary  leads  have  been  connected.  If  the  field  windings  be  not  so  pro\'ided, 
a  bank  of  lamps  or  some  other  non-inductive  resistance  must  be  connected  across  the 
leads  between  the  field  magnet  coils  and  the  point  at  which  the  circuit  is  to  be  op>ened 
and  closed.  This  is  to  pro%-ide  a  path  for  the  discharge  of  the  induced  electromotive 
force.  The  circiiit  should  not  remain  closed  more  than  a  few  seconds  if  the  full  voltage 
can  be  applied.  It  is  well,  however,  to  leave  the  current  on  long  enough  to  run  the 
machine  up  to  about  half  speed  and  make  sure,  by  means  of  a  voltmeter,  that  the  i)olarity 
has  been  corrected.  ^^Tien  this  has  been  ascertained  the  dynamo  should  be  stopped  and 
the  field  winding  leads  returned  to  their  propver  terminals.  Then  the  voltage  will  be 
brought  up  in  the  right  direction,  provided  the  work  has  been  done  correctly. 


OPERA  TION  OF  D  YNA  MOS  587 

This  can  be  tested  by  connecting  a  voltmeter  across  the  terminals  of 
the  armature,  or  by  means  of  a  magnetic  needle  placed  at  a  short  dis- 
tance from  one  of  the  pole  pieces  in  such  a  position  that  it  does  not  point 
to  the  north  pole.  If  the  field  coils  be  improperly  connected,  the  current 
due  to  the  initial  voltage  will  weaken  the  field  magnetism  and  thus  pre- 
vent the  machine  building  up,  and  when  the  field  circuit  is  closed  the 
voltmeter  reading  will  be  reduced,  or  the  magnetic  needle  will  be  less 
strongly  attracted. 

Oues.  What  will  be  the  result  if  the  connections  of 
some  of  the  field  coils  of  a  dynamo  be  reversed  ? 

Ans.  If  one-half  the  number  of  coils  oppose  the  other  half, 
the  field  magnetism  will  be  neutralized  and  the  machine  will  not 
build  np  at  all;  but  if  one  of  the  coils  be  opposed  to  the  others, 
the  machine  might  build  up,  but  the  generated  voltage  will  be 
low,  and  there  will  be  considerable  sparking  at  some  of  the 
brushes. 

Oues.     How  may  it  be  ascertained  which  coil  is  reversed  ? 

Ans.  In  all  dynamos  there  should  be  an  equal  number  of 
positive  and  negative  poles,  and  in  almost  all  of  them  the  poles 
should  be  alternately  positive  and  negative.  Therefore,  if  a 
pocket  compass  be  brought  near  the  pole  pieces,  and  it  show  that 
there  are  more  poles  of  one  kind  than  the  other,  the  indication 
is  that  one  or  more  of  the  coils  are  reversed,  and  the  improper 
sequence  of  alternation  will  determine  which  one  is  wrongly 
connected. 

Oues.  When  a  dynamo  loses  its  residual  magnetism, 
how  can  it  be  made  to  build  up? 

Ans.  By  temporarily  magnetizing  the  field.  To  do  this  a 
current  is  passed  through  it  from  another  dynamo,  or  from 
the  cells  of  a  small  primary  battery.  Usually,  this  will  set  up 
sufficient  initial  magnetism  to  allow  the  machine  to  build  up. 
The  battery  circuit  should  be  broken  before  the  machine  has 
built  up  to  full  voltage. 


588 


HA  WKIXS  ELECTRICITY 


Oues.  What  should  be  done  if  a  dynamo  become 
reversed  by  a  reversal  of  its  field  magnetism  due  to  light- 
ning, short  circuit,  or  othen^ise? 

Ans.  The  residual  magnetism  should  be  reversed  by  a  current 
from  another  dynamo,  or  from  a  batter}-;  but  if  this  be  not 
convenient,  the  connections  between  the  machine  and  the  line 
should  be  crossed  so  that  the  original  positive  terminal  of  the 
d\Tiamo  \^-ill  be  connected  to  the  negative  terminal  of  the  line, 
and  \4ce  versa. 

POSmvE  BUS-BAR  


KE6KnVE  BUS-BVP 


Pig.  679. — Method  of  ccrrec*.:ng  reversed  pclarity  in  campoond  woaad  d>Tiamo.  Tbe  polarity 
may  be  reversed  vriihcui  disconnecting  or  rlianging  the  wire.  The  figure  shows  two 
compound  d>'namos,  and  essentia]  connections.  Tbe  current  from  any  machine  cofmected 
to  the  equalizer  bar  by  its  equalizer  switch  will  di^•ide.  a  portioo  i.<Ning  throasli  the  series 
field  winding  of  the  other  machines  connect*  d  to  the  bus  bar,  tbe  division  being  determined 
by  the  resistance  of  the  different  sets  of  coils.  For  instance,  agoimo  that  No.  1  dynamo 
has  had  its  polarity  reversed  and  that  No.  2  is  running  connected  to  the  has  bar.  The 
method  of  reversing  the  polarity  of  No.  1  machine  is  as  follows:  Xo.  1  macUne  dioald 
be  at  rest  and  then  make  sure  that  tie  circuit  breaker  and  negati\"e  switch  are  open  and 
that  any  other  special  connecticr  s  to  other  machine  or  station  fighting  orcaits  are  open. 
Then  close  ths  positive  and  equalizer  switches,  thus  allowing  a  part  of  the  current  from 
the  other  dynamo  to  pass  through  the  equaHzsr  connecti<xi  and  tfaioagb  the  series  field 
winding  of  No.  1  machine  in  the  usual  direction,  which  will  magnetixe  the  magnetic  ooce. 
If  No.  1  machine  be  a  large  unit  and  No.  2  a  small  unit,  it  will  be  necessary  to  cat  oo: 
the  resistance  of  the  shunt  field  circuits  by  means  of  the  iheostat.  if  it  be  desired  to 
maintain  its  bus  bar  voltage  at  its  normal  point.  This  will  rob  the  series  winding  o: 
any  other  machines  which  may  be  connected  to  the  bus  bars  and  will  lower  the  vxdtage 
slightly.  No.  1  machine  is  then  brought  up  to  full  speed  when  it  wfll  be  foond  to  have 
recovered  its  correct  polarity.  The  positive  switch  may  be  readily  opened,  wattjung  the 
bus  bar  voltage  closely  as  it  will  rise  when  the  current  is  restricted  again  to  the  series 
field  winding  of  the  other  machines.  The  d>Tiamo  will  then  be  ready  to  cnt  in  with  the 
other  machines  as  soon  as  the  voltage  has  been  taxwgfat  up  to  the  pcDper  point,  or  it  may 
be  shut  down  until  required. 


OPERA  TION  OF  D  YNA  MOS  589 

Ques.  Can  a  dynamo  be  reversed  by  reversing  the 
connections  between  the  field  coils  and  the  armature? 

Ans.  No,  for  if  these  connections  be  reversed,  the  machine 
will  not  build  up. 

Ques.     Will  a  dynamo  build  up  if  it  become  reversed? 

Ans.     Yes. 

Ques.    Then  what  is  the  objection  to  a  reversed  dynamo? 

Ans.  Since  the  direction  of  current  of  a  reversed  dynamo  is 
also  reversed,  serious  trouble  may  occur  if  it  be  attempted  to 
connect  it  in  parallel,   with  other  machines  not  reversed. 

Attention  while  Running. — When  a  dynamo  is  started  and 
at  work,  it  will  need  a  certain  amount  of  attention  to  keep  it 
running  in  a  satisfactory  and  efficient  manner.  The  first  point 
to  be  considered  is  the  adjustment  of  the  brushes.  If  this  be 
neglected,  the  machine  will  probably  spark  badly,  and  the 
commutator  and  brushes  will  frequently  require  refitting  to 
secure  good  contact. 

Ques.  What  may  be  said  with  respect  to  the  lead  of 
the  brushes? 

Ans.  The  lead  in  all  good  dynamos  is  very  small,  and  varies 
with  the  load  and  class  of  machine.  The  best  lead  to  give  to  the 
brushes  can  in  all  cases  be  found  by  rotating  the  rocker  and 
brushes  in  either  direction  to  the  right  or  left  of  the  neutral  plane 
until  sparking  commences,  increasing  with  the  movement. 
The  position  midway  between  these  two  points  is  the  correct 
position  for  the  brushes,  for  at  this  position  the  least  sparking 
occurs,  and  it  is  at  this  position  that  the  brushes  should  be 
fixed  by  clamping  the  rocker. 


590 


HAWKINS  ELECTRICITY 


Ques.  How  does  the  lead  vary  in  the  different  types  of 
dynamo? 

Ans.  In  series  dynamos  giving  a  constant  current,  the 
brushes  require  practically  no  lead.  In  shunt  and  compound  dy- 
namos the  lead  varies  with  the  load,  and  therefore  the  brushes 
must  be  rotated  in  the  direction  of  rotation  of  the  armature  with 
an  increase  of  load,  and  in  the  opposite  direction  with  a  decrease 
of  load. 


Fig.  6S0. — Method  of  taking  temperature.  In  taking  the  temperature  of  a  hot  part,  it  is  con- 
venient to  use  a  thermometer  in  which  the  scale  of  degrees  has  been  etched  on  the  stem. 
Bind  this  to  the  heated  part,  ha^•ing  first  taken  the  precaution  to  cover  the  bulb  with 
waste  to  prevent  the  radiation  of  heat  and  take  the  reading  when  the  column  of  mercury 
has  ceased  to  rise.  The  question  which  most  often  presents  itself  to  the  attendant  is  hoiv 
hot  can  the  various  parts  of  a  dynamo  or  motor  become  and  yet  be  within  the  safe  limit. 
The  degree  of  heat  can  be  determined  by  applWng  the  hand  to  the  various  parts.  If  tha 
heat  be  bearable  it  is  entirely  harmless,  but  if  the  heat  become  unbearable  to  the  hand  for 
more  than  a  few  seconds,  the  safety  limit  has  been  reached  and  the  machine  should  be 
stopped  and  the  fault  located.  Of  course  when  the  solder  begins  to  melt  at  the  commutator 
connections  and  shellac  begins  to  "fr\-  out "  of  the  armature  and  an  odor  of  burnt  cotton 
begins  to  pervade  the  air.  the  safe  limit  has  been  far  e.xceeded.  and  in  most  cases,  as  a 
matter  of  fact,  serious  damage  is  the  result.  To  be  more  definite,  no  part  of  the  dynamo 
or  motor  should  be  aUo^ved  to  rise  in  temperature  more  than  SO  degrees  F.  above  the  temperature 
of  the  surrounding  air.  excepting  in  the  case  of  commutators  where  no  solder  has  been 
used  to  connect  the  leads.    These  can  be  allowed  to  rise  to  a  still  higher  temperature. 


In  cases  where  the  dynamos  are  subjected  to  a  rapidly  var^'ing  or 
fluctuating  load,  it  is  of  course  not  possible  to  constantly  shift  the 
brushes  as  the  load  varies,  therefore  the  brushes  should  be  fixed  in  the 
positions  where  the  least  sparking  occurs  at  the  moment  of  adjustment. 


OPERA  TION  OF  D  YNA  MOS 


591 


If  at  any  time  violent  sparking  occur,  which  cannot  be  reduced  or 
suppressed  by  varying  the  position  of  the  brushes  by  rotating  the 
rocker,  the  machine  should  be  shut  down  at  once,  otherwise  the  com- 
mutator and  brushes  are  liable  to  be  destroyed,  or  the  armature  burnt 
up.    This  especially  refers  to  high  tension  machines. 

Oues.  What  should  be  done  if  the  brushes  begin  to 
spark  excessively  ? 

Ans.  First,  look  at  the  ammeter  to  see  if  an  excessive  amount 
of  current  is  being  delivered;  second,  see  if  the  brushes  make 
good  contact  with  the  commutator,  and  if  the  latter  have  a  bar 
too  high,  or  too  low,  and  an  open  circuit. 


Figs.  681  and  682. — Remedies  for  leakage  of  oil  from  self-oiling  bearings.  If  there  be  sufficient 
space,  a  metal  ring  may  be  attached  to  the  shaft  as  in  fig.  681.  With  this  arrangement 
the  high  speed  of  the  shaft  will  carry  the  oil  outside  of  the  ring  and  throw  it  off  in  the  oil 
reservoir.  Another  way  is  to  insert  a  tin  apron,  as  shown  in  fig.  682  at  T,  which  will  serve 
to  drain  the  oil  which  may  creep  along  the  shaft,  and  also  cut  off  the  draft  from  the  pulley 
which  may  suck  the  oil  out  of  the  bearing.  Sometimes  a  tin  fan  is  attached  to  the  pulley, 
which  tends  to  drive  the  oil  back  into  the  bearing,  and  which  also  assists  in  keeping  the 
bo.\  cool. 


Oues.     What  should  be  done  if  the  current  be  excessive  ? 

Ans.  If  the  current  exceed  the  rated  capacity  by  more  than 
50  per  cent.,  and  continue  for  more  than  a  few  minutes,  the 
main  switch  should  be  opened,  otherwise  the  machine  may  be 
seriously  injured. 


592 


HAWKINS  ELECTRICITY 


Oues.     How  does  an  excessive  current  injure  a  dynamo? 

Ans.  By  causing  it  to  overheat  which  destroys  the  insulation 
of  the  armature,  commutator,  etc. 

Lubrication. — The  shaft  bearings  of  dynamos  may  be 
lubricated  by  sight  feed  oilers  or  oil  rings.  The  latter  method  is 
almost  universally  used.  An  oil  well  is  provided  in  the  hollow 
casting  of  the  pedestals  as  shown  in  fig.  728.    Oil  rings  revolve 


Fig.  683. — Imaginative  view  of  a  shaft  showing  its  rough  granular  structure.  In  operation 
these  minute  irregularities  interlock  and  act  as  a  retarding  force,  or  frictional  resistance. 
Hence,  the  necessity  for  lubrication — a  lubricant  presents  a  thin  intervening  film  against 
which  the  surfaces  rub. 


with  the  shaft  and  feed  the  latter  with  oil,  which  is  contin- 
uously brought  up  from  the  reservoir  below.  The  dirt  settles  to 
the  bottom  and  the  upper  portion  of  the  oil  remains  clear  for  a 
long  period,  after  which  it  is  drawn  off  through  the  spigot  and  a 
fresh  supply  poured  in  through  openings  provided  in  the  top. 
The  latter  are  often  located  directly  over  the  slots  in  which  the 
rings  are  placed,  so  that  the  bearings  can  be  lubricated  directly 
by  means  of  an  oil  cup,  if  the  rings  fail  to  act  or  the  reservoir 
become  exhausted. 


OPERATION  OF  DYNAMOS  593 

Oues.  What  kind  of  oil  can  should  be  used  in  filling  the 
reservoir,  or  oil  cups? 

Ans.  One  made  of  some  non-magnetic  material  such  as 
copper,  brass,  or  zinc. 

If  iron  cans  be  used,  they  are  liable  to  be  attracted  by  the  field  magnets, 
and  thus  possibly  catch  in  the  armature. 

Oues.  What  is  the  indication  of  insufficient  lubri- 
cation? 

Ans.     The  bearings  become  unduly  heated. 

Oues.  What  precaution  should  be  taken  with  new 
dynamos? 

Ans.  They  are  liable  to  heat  abnormally  and  for  the  first 
few  days  they  should  be  carefully  watched  and  liberally  sup- 
plied with  oil. 

After  a  dynamo  has  been  running  for  a  short  time  under  full  load,  its 
armature  imparts  a  certain  amount  of  heat  to  the  bearings,  a  little  more 
also  to  the  bearing  on  the  commutator  end  of  shaft;  beyond  this  there 
is  no  excuse  for  excessive  heating.  The  latter  may  result  from  various 
causes,  some  of  which  are  given  with  their  remedies,  as  follows: 

1.  A  poor  quality  of  oil,  dirty  or  gritty  matter  in  the  oil; 

2.  Journal  boxes  too  tight; 

3.  Rough  journals,  badly  scraped  boxes; 

4.  Belt  too  tight;      _ 

5.  Bearings  out  of  line; 

6.  Overloaded  dynamo; 

7.  Bent  armature  shaft. 

Oues.     What  is  the  allowable  degree  of  heating? 

Ans.  It  may  be  taken  as  a  safe  rule  that  no  part  of  a  working 
dynamo  should  have  a  temperature  of  more  than  80°  Fahr.  above 
that  of  the  surrounding  air. 

Accordingly,  if  the  temperatur&  of  the  engine  room  be  noted  before 
applying  the  thermometer  to  the  machine,  it  can  at  once  be  seen  if  the 
latter  be  working  at  a  safe  temperature.     In  taking  the  temperature. 


594 


HAWKINS  ELECTRICITY 


the  bulb  of  the  thermometer  should  be  wrapped  in  a  woolen  rag.  The 
screws  and  nuts  securing  the  different  connections  and  cables  should  be 
examined  occasionally,  as  they  frequently  work  loose  through  vibration. 


Instructions  for  Stopping  Dynamos. — When  shutting 
down  a  machine,  the  load  should  first  be  gradually  reduced,  if 
possible,  by  easing  down  the  engine;  then  when  the  machine  is 
supplying  little  or  no  current,  the  main  switch  should  be  opened. 
This  reduces  the  sparking  at  the  switch  contacts,  and  prevents 
the  engine  racing. 


Fig.  684. — Diagram  illustrating  forces  acting  on  a  dynamo  armature.  In  the  figure  the  normal 
field  magneto-motive  force  is  in  the  direction  of  the  line  1 .  2,  produced  by  the  field  circuit 
G,if  there  were  no  current  in  the  armature.  But  as  soon  as  the  armature  current  flows, 
it  produces  the  opposing  force  3,  4,  which  must  be  combined  with  1,  2  to  give  the 
resulting  force  to  produce  magnetism  and  hence  voltage.  The  resultant  1,  .^,  if  3,  4  be 
large  enough,  does  not  differ  much  from  the  original  force  1.  2.  Or,  e.xpressed  in  a  more 
physical  way,  the  brushes  E,  F,  rest  on  the  commutator  and  all  the  turns  embraced  by  twice 
the  angle  6.  3,  F,  oppose  the  flow  of  flux  through  the  armature  core  as  well  as  all  the  turns 
embraced  by  twice  the  angle,  7,  3,  E.  The  remaining  turns  distort  the  flux,  making  the 
pole  comers  at  A  and  B  denser,  and  at  C  and  D  rarer.  So  that  all  the  effect  is  to  kill  an 
increase  of  flux,  or  voltage.  This  cross  magnetism  tends  also  to  decrease  the  flow  of 
flux,  for  the  extra  ampere  turns  required  to  force  the  flux  through  the  dense  pole  tips  are 
greater  than  the  decreased  ampere  turns  relieved  by  the  reduction  of  flux  at  the  other  pole 
tips;  this  follows,  since  iron  as  it  increases  in  magnetic  density  requires  ampere  turns  greater 
in  proportion  than  the  increase  of  flux. 


When  the  voltmeter  almost  indicates  zero,  the  brushes  should 
be  raised  from  contact  with  the  commutator.  This  prevents  the 
brushes  being  damaged  in  the  event  of  the  engine  making  a 
backward  motion,  which  it  often  does,  particularly  in  the  case  of 


OPERA  TION  O  F  D  YNA  MOS  595 

a  gas  engine.  On  no  account,  however,  should  the  brushes  be 
raised  from  the  commutator  while  the  machine  is  generating  any 
considerable  voltage;  for  not  only  is  the  insulation  of  the  ma- 
chine liable  to  be  damaged,  but  in  the  case  of  large  shunt 
dynamos,  the  person  lifting  the  brushes  is  liable  to  receive  a 
violent  shock. 

Oues.  What  attention  should  the  machine  receive  after 
it  has  been  shut  down? 

Ans.  It  should  be  thoroughly  cleaned.  Any  adhering  copper 
dust,  dirt,  etc.,  should  be  removed  from  the  armature  by  dusting 
with  a  stiff  brush,  and  the  other  portions  of  the  machine  should 
be  thoroughly  cleaned  with  linen  rags.  Waste  should  not  be 
used,  as  it  is  liable  to  leave  threads  or  fluff  on  the  projecting 
parts  of  the  machine,  and  on  the  windings  of  the  armature, 
which  is  difficult  to  remove. 

Oues.  What  attention  should  be  given  to  the  brushes 
and  brush  gear? 

Ans.  They  should  be  examined  and  thoroughly  cleaned.  If 
necessary  the  brushes  should  be  refitted  and  readjusted.  All 
terminals,  screws,  bolts,  etc.,  should  be  carefully  cleaned  and 
screwed  up  ready  for  the  next  run.  The  brush  holders  should 
receive  special  attention,  as  when  dirty,  they  are  liable  to  stick 
and  cause  sparking.  All  dirt  and  oil  should  be  removed  from  the 
springs,  contacts,  pivots,  and  other  working  parts. 

It  is  advisable  at  stated  intervals  to  entirely  remove  the  brush  holders 
from  the  rocker  arms,  and  give  them  a  thorough  cleaning  by  taking 
them  to  pieces,  and  cleaning  each  part  separately  with  emery  cloth  and 
benzoline  or  soda  solution. 

Another  point  to  which  particular  attention  should  be  given  is  the 
cleaning  of  the  brush  rocker.  This  being  composed  wholly  of  metal, 
and  the  two  sets  of  positive  and  negative  brushes  being  only  separated 
from  it  by  a  few  thin  insulating  washers,  it  follows  that  if  any  copper 
dust  given  off  by  the  brushes  be  deposited  in  the  neighborhood  of  these 
washers,  there  is  considerable  liability  for  a  short  circuit  of  the  machine 
to  occur  by  the  dust  bridging  across  the  insulation. 


596  HAWKINS  ELECTRICITY 

Oues.    What  further  attention  should  be  given? 

Ans.  It  is  a  good  plan,  when  the  machine  has  been  thor- 
oughly cleaned  and  all  connections  made  secure,  to  occasionally- 
test  the  insulation  of  the  different  parts.  If  a  record  be  kept  of 
these  tests,  any  deterioration  of  the  insulation  can  at  once  be 
detected,  localized  and  remedied  before  it  has  become  suffi- 
ciently bad  to  cause  a  breakdown. 

As  a  means  of  protecting  the  machine  from  any  moisture,  dirt,  etc., 
while  standing  idle,  it  is  advisable  to  cover  it  with  a  suitable  waterproof 
cover. 


COUPLING  OF  DYNAMOS  597 


CHAPTER  XXX 
COUPLING   OF  DYNAMOS 


Series  and  Parallel  Connections. — When  it  is  necessary 
to  generate  a  large  and  variable  amount  of  electrical  energy,  as 
must  be  done  in  central  generating  stations,  apart  from  the 
question  of  liability  to  breakdown,  it  is  neither  economical  nor 
desirable  that  the  whole  of  the  energy  should  be  furnished  from 
a  single  dynamo.  Since  the  efficiency  of  a  dynamo  is  dependent 
upon  its  output  at  any  moment,  or  the  load  at  which  it  is  worked 
(the  efficiency  varying  from  about  95  per  cent,  at  full  load  to 
80  per  cent,  at  half  load),  it  is  advisable  m  order  to  secure  the 
greatest  economy  in  working,  to  operate  any  dynamo  as  near 
full  load  as  possible. 

Under  the  above  circumstances,  when  the  whole  of  the  output  is 
generated  by  a  single  dynamo  this  can  evidently  not  be  eflfected,  for  the 
load  will  naturally  fluctuate  up  and  down  during  the  working  hours,  as 
the  lamps,  motors,  etc.,  are  switched  into  and  out  of  circuit;  hence, 
although  the  dynamo  may  be  working  at  full  load  during  a  certain 
portion  of  the  day,  at  other  times  it  may  probably  be  working  below 
half  load,  and  therefore  the  efficiency  and  economy  in  working  in  such 
an  arrangement  is  very  low. 

Ques.  How  is  maximum  efficiency  secured  with  vari- 
able load  ? 

Ans.  It  is  usual  to  divide  up  the  generating  plant  into  a 
number  of  units,  varying  in  size,  so  that  as  the  load  increases, 
it  can  either  be  shifted  to  machines  of  larger  size,  or  when  it 
exceeds  the  capacity  of  the  largest  dynamo,  the  output  of  one 


598  HAWKINS  ELECTRICITY 

can  be  added  to  that  of  another,  and  thus  the  dynamos  actually 
at  work  at  any  moment  can  be  operated  as  nearly  as  possible  at 
full  load. 

Oues.  What  should  be  noted  with  respect  to  connecting 
one  dynamo  to  another? 

Ans.  It  is  necessary  to  take  certain  precautions  (as  later 
explained)  in  order  that  the  other  dynamos  may  not  be  affected 
by  the  change,  and  that  they  may  work  satisfactorily  together. 

Oues.  What  are  the  two  methods  of  coupling 
dynamos? 

Ans.     They  are  connected  in  series,  or  in  parallel. 

In  coupling  dynamos  in  series,  the  current  capacity  of  the  plant  is 
kept  at  a  constant  value,  while  the  output  is  increased  in  proportion  to 
the  pressures  of  the  machines  in  circuit. 

When  connected  in  parallel,  the  pressujes  of  all  the  machines  are  kept 
at  a  constant  value,  while  the  output  of  the  plant  is  increased  in  pro- 
portion to  the  current  capacities  of  the  machines  in  circuit. 

Coupling  Series  Dynamos  in  Series. — Series  wound  dyna- 
mos will  run  satisfactorily  together  without  special  precautions 
when  coupled  in  series,  if  the  connections  be  arranged  as  in  fig. 
685. 

The  positive  terminal  of  one  dynamo  is  connected  to  the  negative 
terminal  of  the  other,  and  the  two  outer  terminals  are  connected  directly 
to  the  two  main  conductors  or  bus  bars  through  the  ammeter  A,  fuse  F, 
and  switch  S.  If  it  be  desired  to  regulate  the  ]»ressure  and  output  of  the 
machines,  variable  resistances,  or  hand  regulators  R,  R*,  may  be  ar- 
ranged as  shunts  to  the  series  coils  as  shown,  so  as  to  divert  a  por- 
tion or  the  whole  of  the  current  therefrom. 

Series  Dynamos  in  Parallel. — Simple  series  wound  dynamos 
not  being  well  adapted  for  the  purpose  of  maintaining  a  constant 
pressure,  are  in  practice  seldom  coupled  in  parallel;  the  condi- 
tions or  working,  however,  derive  importance  from  the  fact  that 


COUPLING  OF  DYNAMOS 


599 


compound  dynamos,  being  provided  with  series  coils,  arc  subject 
to  similar  conditions  when  working  in  parallel,  which  is  frequently 
the  case. 

Oues.    What  may  be  said  with  respect  to  coupling  two 
or  more  plain  series  dynamos  in  parallel? 

Ans.     The  same  procedure  cannot  be  followed  as  in  the  case 
of  plain  shunt  dynamos,  for  the  reason  that  if  the  voltage  of  the 


Fig.  685. — Diagram  showing  method  of  coupling  series  dynamos  in  series.  R  and  R'  are  two 
hand  regulators  which  are  placed  in  shunt  across  the  coil  terminals  to  regulate  the  pressure 
and  output  of  the  machine. 


dynamo  to  be  coupled  be  exactly  equal  to  that  of  the  bus  bars 
when  connected  in  parallel,  the  combination  will  be  unstable. 

Oues.     Why  is  this? 

Ans.  If,  from  any  cause,  the  pressure  at  the  terminals  of  one 
of  the  dynamos  fall  below  that  of  the  others,  it  immediately 
takes  a  smaller  proportion  of  the  load;    as  a  consequence,  the 


600 


HAWKINS  ELECTRICITY 


ctirrent  in  its  field  coils  is  reduced,  and  a  fvirther  fall  of  pressure 
immediately  takes  place.  This  again  causes  the  dynamo  to  re- 
linquish a  portion  of  its  load,  and  again  occurs  a  further  fall  of 
pressure.  Thus  the  process  goes  on,  until  finally  the  dynamo 
ceases  to  supply  current,  and  the  current  from  the  other  dyna- 
mos flowing  in  its  field  coils  in  the  reverse  direction  reverses  its 
magnetism,  and  causes  it  to  run  as  a  motor  against  the  dri\'ing 
power  in  the  opposite  direction  to  that  in  which  it  previously 
ran  as  a  dynamo. 

--f 


Fig.  686. — Diagram  showing  method  of  coupling  series  dynamos  £n  i)aralleL     In  the  diagram 
A,  A',  are  ammeters  ;  F,  F',  fuses  ;  S,  S',  switches. 

Under  such  circumstances  the  armature  is  liable  to  be  destroyed  if 
the  fuse  be  not  immediately  blown,  and  in  any  case  it  is  subjected  to 
a  very  detrimental  shock.  This  tendency  to  reverse  in  series  dynamos 
can  be  effectually  prevented  by  connecting  the  field  coils  of  all  the 
dynamos  in  parallel. 


Oues.     How  are  the  field  coils  of  all  the  dynamos  con-   ^ 
nected  in  parallel? 

Ans.     This  is  effected  in  practice  by  connecting  the  ends  of  all 
the  series  coils  where  they  join  on  to  the  armattire  circuit  by  a 


CO  UPLING  OF  D  YNA  MOS  601 

third  connection,  called  the  "  equalizing  connection,"  or  "  equal- 
izer," as  shown  in  fig.  686. 

Ques.     What  is  the  effect  of  the  equalizer? 

Ans.  The  immediate  effect  is  to  cause  the  whole  of  the  current 
generated  by  the  plant  to  be  divided  among  the  series  coils  of  the 
several  dynamos  in  the  inverse  ratio  of  their  resistance,  without 
any  regard  as  to  whether  this  current  comes  from  one  armature, 
or  is  divided  among  the  whole.  The  fields  of  the  several  dynamos 
being  thus  maintained  constant,  or  at  any  rate  being  caused 
to  vary  equally,  the  tendency  for  the  pressure  of  one  dynamo 
to  fall  below  that  of  the  others  is  diminished. 

Shunt  Dynamos  in  Series. — The  simplest  operation  in 
connection  with  the  coupling  of  dynamos,  and  the  one  used  prob- 
ably more  frequently  in  practice  than  any  other,  is  the  coupling 
of  two  or  more  shunt  dynamos  to  run  either  in  series  or  in  parallel. 
When  connected  in  series,  the  positive  terminal  of  one  machine 
is  joined  to  the  negative  of  the  other,  and  the  two  outer  terminals 
are  connected  through  the  ammeter  A,  fuses  F,  F',  and  switch 
S,  to  the  two  main  conductors  or  omnibus  bars  as  represented  in 
fig.  687.  The  machine  will  operate  when  the  connections  are 
arranged  in  this  manner,  if  the  ends  of  the  shunt  coils  be  con- 
nected to  the  terminals  of  their  respective  machines. 

Shunt  Dynamos  in  Parallel. — The  coupling  of  two  or  more 
shunt  dynamos  to  run  in  parallel  is  effected  without  any  difficulty. 
This  method  of  coupling  dynamos  is  one  that  is  very  frequently 
used.  Fig.  688  illustrates  diagrammatically  the  method  of 
arranging  the  connections.  The  positive  and  negative  terminals 
of  each  machine  are  connected  respectiv^ely  to  two  massive 
insulated  copper  bars,  shown  at  the  top  of  the  diagram,  called 
omnibus  bars,   through   the   double  pole  switches,  S,  S',  and 


602 


HAWKINS  ELECTRICITY 


the  double  pole  fuses  F,  F'.  Ammeters,  A,  A'  are  inserted 
in  the  main  circuit  of  each  machine,  and  sen^e  to  indicate  the 
amoimt  of  current  generated  by  each.  An  automatic  sv^-itch  or 
cutout,  Ac,  Ac',  is  also  shown  as  being  included  in  the  main 
circuit  of  each  of  the  machines,  although  this  appliance  is  some- 
times dispensed  ^\'ith.  The  pressure  of  each  of  the  machines  is 
regulated  independently  by  means  of  the  hand  regulators  R,  R', 
inserted  in  series  vrith  the  shunt  circuit. 


BUS     BARS 


^vwww — vwww^ 


IG.  687. — Diagram  showing  method  of  coupling  shunt  djTiamos  in  series.  The  ends  of  the 
shunt  coils  may  be  connected  to  the  terminals  of  their  respective  machine,  or  they  may  bo 
connected  in  series  as  shown. 

The  shunt  circuits  are  represented  as  being  connected  to  the  positi%'e 
and  negative  terminals  of  the  respective  machines,  but  in  many  cases 
where  the  load  is  subjected  to  sudden  variations,  and  when  a  large 
number  of  machines  is  connected  to  the  bus  bars,  the  shunt  coils  are 
frequently  connected  direct  to  these.  In  such  circumstances  this 
method  is  preferable,  as  by  means  of  it  the  fields  of  the  idle  dynamos 
can  be  excited  almost  at  once  direct  from  the  bus  bars  by  the  current 
from  the  working  dynamos;  hence,  if  a  heavy  ]oad  come  on  suddenly, 
no  time  need  be  lost  in  building  up  a  new  machine  previous  to  switching 
it  into  parallel.    The  pressure  of  the  lamp  circuit  is  given  by  a  voltmeter. 


COUPLING  OF  DYNAMOS 


6U3 


whose  terminals  are  placed  across  the  bus  bars;  and  the  pressure  at  the 
terminals  of  each  of  the  machines  is  indicated  by  separate  voltmeters 
or  pilot  lamps,  the  terminals  of  which  are  connected  to  those  of  the 
respective  machines. 

Oues.     Describe  a  better  method  of  parallel  connection. 

Ans.  Better  results  are  obtained  by  connecting  both  the 
shunt  coils  in  series  with  one  another,  so  that  they  form  one  long 
.siiunt  between  the  two  main  conductors,  the  same  as  in  fig.  687. 

BUS      B^RS 


Fig.  688. — Diagram  showing  method  of  coupling  shunt  dynamos  in  parallel. 

When  arranged  in  this  way,  the  regulation  of  both  machines  may  be 
effected  simultaneously  by  inserting  a  hand  regulator  (R)  in  series  with 
the  shunt  circuit  as  represented. 

Switching  Dynamo  Into  and  Out  of  Parallel. — In  order 

to  put  an  additional  dynamo  in  parallel  with  those  already  work- 
ing, it  is  necessary  to  run  the  new  dynamo  up  to  full  speed,  and, 
when  it  excites,  regulate  the  pressure  by  means  of  a  hand  regu- 
lator until  the  voltmeter  connected  to  the  terminals  of  the 
machines  registers  one  or  two  volts  more  than  the  voltme<" 


604  HAWKINS  ELECTRICITY 

connected  to  the  lamp  circmt,  and  then  close  the  switch.  The 
load  upon  the  machine  can  then  be  adjusted  to  correspond  w-ith 
that  upon  the  other  machines  by  means  of  the  hand  regulator. 

Oues.  In  connecting  a  shunt  dynamo  to  the  bus  bars, 
must  the  voltage  be  carefully  adjusted? 

Ans.  There  is  little  danger  in  overloading  the  armature  in 
making  the  connection  hence  the  pressure  need  not  be  accurately 
adjusted. 

It  is,  in  fact,  common  practice  in  central  stations  to  judge  the  voltage 
of  the  new  dynamo  merely  by  the  appearance  of  its  pilot  lamp. 

Oues.     How  is  a  machine  cut  out  of  the  circuit? 

Ans.  WTien  shutting  dowTi  a  machine,  the  load  or  cturent 
must  first  be  reduced,  by  gradually  closing  the  stop  valve  of  the 
engine,  or  inserting  resistance  into  the  shunt  circuit  by  means  of 
the  hand  regulator;  then  when  the  ammeter  indicates  nine  or 
ten  amperes,  the  main  switch  is  opened,  and  the  engine  stopped. 

By  following  this  plan,  the  heavy  sparking  at  the  switch  contacts  is 
avoided,  and  the  tendency  for  the  engine  to  race,  reduced. 

Oues.  What  precaution  must  be  taken  in  reducing  the 
current? 

Ans.  Care  must  be  taken  not  to  reduce  the  current  too 
much. 

Oues.    Why  is  this  necessary? 

Ans.  There  is  danger  that  the  machine  may  receive  a  reverse 
current  from  the  other  dynamos,  resulting  in  hea\^''  sparking  at 
the  commutator,  and  in  the  machine  being  driven  as  a  motor. 

Oues.    What  provision  is  made  to  obviate  this  danger? 

Ans.  Dynamos  that  are  to  be  run  in  parallel  are  frequently 
provided  with  automatic  cutouts,  set    so  as  to  automatically 


COUPLING  OF  DYNAMOS  605 

switch  out  the  machine  when  the  current  falls  below  a  certain 
minimum  value. 

Dividing  the  Load. — If  a  plant,  composed  of  shunt  dynamos 
running  in  parallel,  be  subjected  to  variations  of  load,  gradual 
or  instantaneous,  the  dynamos  will,  if  they  all  have  similar 
characteristics,  each  take  up  an  equal  share  of  the  load.  If, 
however,  as  is  sometimes  the  case,  the  characteristics  of  the 
dynamos  be  dissimilar,  the  load  will  not  be  shared  equally,  the 
dynamos  with  the  most  drooping  characteristics  taking  less  than 
their  share  with  an  increase  of  load,  and  more  than  their  share 
with  a  decrease  of  load.  If  the  difference  be  slight,  it  may  be 
readily  compensated  by  means  of  the  hand  regulator  increasing 
or  decreasing  the  pressures  of  the  machines,  as  the  load  varies. 
If,  however,  the  difference  be  considerable,  and  the  fluctuations 
of  load  rapid,  it  becomes  practically  impossible  to  evenly  divide 
the  load  by  this  means. 

Under  such  circumstances,  the  pressure  at  the  bus  bars  is 
liable  to  great  variations,  and  there  is  also  liability  of  blowing  the 
fuses  of  the  overloaded  dynamos,  thus  precipitating  a  general 
breakdown.  To  cause  an  equal  division  of  the  load  among  all  the 
dynamos,  under  such  circumstances,  it  is  needful  to  insert  a  small 
resistance  in  the  armature  circuits  of  such  dynamos  as  possess 
the  straightest  characteristics,  or  of  such  dynamos  as  take  more 
than  their  share  of  an  increase  of  load.  By  suitably  adjusting 
or  proportioning  the  resistances,  the  pressures  at  the  terminals 
of  all  the  machines  may  be  made  to  vary  equally  under  all  varia- 
tions of  load,  and  each  of  the  machines  will  then  take  up  its 
proper  share  of  the  load. 

Coupling  Compound  Dynamos  in  Series. — Since  com- 
pound dynamos  may  be  regarded  as  a  combination  of  the  shunt 
and  series  wound  machines,  and  as  no  special  difficulties  are 

\ 


GOG 


HAWKINS  ELECTRICITY 


encoiintered  in  ninning  these  latter  in  series,  analogy  at  once 
leads  to  the  conclusion  that  compound  dynamos  under  similar 
circumstances  may  be  coupled  together  with  equal  facihty. 

Ques.     How    are    compound    dynamos    connected    to 
operate  in  series? 

Ans.     The  series  coils  of  each  are  connected  as  in  fig.  685,  and 
the  shunt  coils  are  connected  as  a  single  shunt  as  in  fig.  687,  which 


—    I 


3v'S  BARS 


RHEOSTAT 


■f 


,  .  SHUNT - 


Fig.  6S9. — Coupling  compound  djiiamos  in  series;  short  shunt  connection, 
indicate  the  changes  that  would  be  made  for  long  shunt  connection. 


The  dotted  lines 


may  either  extend  simply  across  the  outer  brushes  of  the  machines, 
so  as  to  form  a  double  short  shtmt,  or  may  be  a  shunt  to  the  bus 
bars  of  external  circuit,  so  as  to  form  a  double  long  shimt. 


Compound  Dynamos  in  Parallel. — Machines  of  this  type 
will  not  run  satisfactorily  together  in  parallel  unless  all  the 
series  coils  are  connected  together  by  an  equaUzing  connection, 
as  in  series  dynamos.    The  method  of  arranging  the  connections 


CO  UP  LING  OF  D  YNA  MOS 


607 


as  adopted  in  practice,  being  illustrated  in  fig.  690.  By  means 
of  it  idle  machines  are  completely  disconnected  from  those  at 
work. 

Oues.     How  is  the  equalizer  connected? 

Ans.  The  equalizer  is  connected  direct  to  the  positive  brushes 
of  all  the  dynamos,  a  three  pole  switch  being  fitted  for  discon- 
necting it  from  the  circuit  when  the  machine  to  which  it  is  con- 


ZER 

NO.l  DYNAMO 
SHUNT  COILS 


SHUNT  COILS '-p"  SHUNT  COILS  n[ 


Fig.  690. — Diagram  showing  method  of  coupling  compound  dynamos  in  parallel. 

nected  is  not  working.  The  two  contacts  of  the  switch  are  re- 
spectively connected  to  the  positive  and  negative  conductors, 
while  the  central  contact  is  connected  to  the  equalizer. 


Switching  a  Compound  Dynamo  Into  and  Out  of  Parallel. 

— If  the  characteristics  of  all  the  dynamos  be  similar,  and  the 
coimections  arranged  as  in  figs.  690,  or  691,  the  only  precaution 


608 


HAWKINS  ELECTRICITY 


to  be  observed  in  switching  a  new  machine  into  parallel  is  to 
have  its  voltage  equal,  or  nearly  equal  to  that  of  the  bus  bars 
previous  to  closing  the  switch.  If  this  be  the  case,  the  new 
machine  will  take  up  its  due  share  of  the  load  without  any  shock. 

Ques.     How  is  a  compound  dynamo,  running  in  parallel, 
cut  out  of  circuit? 

Ans.     The  load  is  first  reduced  to  a  few  amperes,  as  in  the  case 
of  shunt  dynamos,   either  by  easing  down  the  engine,  or  by 


SHUNT  COILS      r^  SHUNT  COILS    M 


Fig.  091. — Diagram  showing  another  and  better  method  of  coupling  compound  dynamos  in 
parallel.  With  this  arrangement  the  idle  machines  are  completely  disconnected  from  those 
at  work.  The  same  reference  letters  are  common  in  both  diagrams.  S,  S'  are  switches; 
F,  F'  fuses;  A,  A'  ammeters,  which  indicate  the  total  amount  of  current  generated  by  each 
of  the  machines;  AC,  AC,  automatic  switches,  arranged  for  automatically  switching  out 
a  machine  in  the  event  of  the  pressure  at  its  terminals  being  reduced  through  any  cause; 
R.  R'  are  hand  regulators,  inserted  in  the  shunt  circuits  of  each  of  the  machines,  by  means 
of  which  the  pressures  of  the  individual  machines  may  be  varied  and  the  load  upon  each 
adjusted.  The  pressure  at  the  bus  bars  is  given  by  the  voltmeter  V,  one  terminal  of  which 
is  connected  to  each  of  the  bars;  a  second  voltmeter  may  be  used,  to  give  the  pressure  of 
any  individual  machine,  by  connecting  "voltmeter  keys"  to  the  terminals  of  each  of  the 
machines,  or  a  separate  voltmeter  may  be  used  for  each  individual  machine.  The  only 
essential  difference  between  figs.  (190  and  691  is.  that  in  fig.  690  the  equalizer  is  connected 
direct  to  the  positive  brushes  of  all  the  dynamos,  while  in  fig.  C91  the  equalizer  is  brought 
up  to  the  switchboard  and  arranged  between  the  two  bus  bars,  a  switch  being  fitted  for 
disconnecting  it  from  the  circuit  when  the  machine  to  which  it  is  connected  is  not  working. 


CO UPLING  OF  D  YNA  MOS  609 

cutting  resistance  into  the  shunt  circuit  by  means  of  the  hand 
regulator,  and  then  opening  the  switch.  Previous  to  this, 
however,  it  is  advisable  to  increase  the  voltage  at  the  bus  bars  to 
a  slight  extent,  as  while  slowing  down  the  engine  the  load  upon 
the  outgoing  dynamo  is  transferred  to  the  other  dynamo  arma- 
tures, and  the  current  in  their  series  coils  not  being  increased  in 
proportion,  the  voltage  at  the  bus  bars  is  consequently  reduced 
somewhat. 


Equalizing  the  Load. — When  a  number  of  compound  dyna- 
mos of  various  output,  size,  or  make,  are  running  together  in 
parallel,  it  frequently  happens  that  all  their  characteristics  are 
not  exactly,  similar,  and  therefore  the  load  is  unequally  dis- 
tributed, some  being  overloaded,  while  others  do  not  take  up 
their  proper  share  of  the  work. 


NOTE. — The  action  of  an  equalizing  bar  in  equalizing  the  load  on  compound  dynamos  run 
in  parallel  may  be  explained  as  follows:  'I'he  compound  winding  of  a  dynamo  raises  the  pressure 
in  proportion  to  the  current  flowing  through  it,  and  if ,  in  a  system  of  parallel  operated  compound 
dynamos  without  the  equaUzing  connection,  the  current  given  by  one  machine  were  slightly 
greater  than  the  currents  from  the  others,  the  pressure  of  that  machine  would  increase.  With 
this  increase  in  pressure  above  the  other  machines,  a  still  greater  current  would  flow,  and  so 
raise  the  pressure  further.  The  effect  is  therefore  cumulative,  and  in  time  the  one  dynamo 
would  be  carrying  too  great  a  proportion  of  the  whole  current  of  the  system.  With  the  equal- 
izing connection,  whatever  the  current  flowing  from  each  machine,  the  currents  in  the  various 
compound  windings  are  all  equal,  and  so  the  added  pressure  due  to  the  compound  winding  is 
practically  the  same  in  each  machine.  Any  inequality  in  output  from  the  machines  is  readily 
eliminated  by  adjusting  the  shunt  currents  by  means  of  the  shunt  rheostats.  When  compound 
wound  dynamos  are  operated  in  parallel,  the  equalizer  bar  insures  uniform  distribution  among 
the  series  coils  of  the  machines. 

NOTE. — To  secure  the  best  results  in  parallel  operation,  dynamos  should  be  of  the  same 
design  and  construction  and  should  possess  as  nearly  as  possible  the  same  characteristics;  that  is, 
each  should  respond  with  the  same  readiness,  and  to  the  same  extent,  to  any  change  in  its  field 
excitation.  Any  number  of  such  machines  may  be  operated  in  parallel.  1  he  usual  practice  is 
to  connect  the  equalizer  and  the  series  field  to  the  positive  terminal,  though  if  desired,  they 
may  be  connected  to  the  negative  terminal ;  both  however,  must  be  connected  to  the  same 
terminal.  The  resistance  of  the  equaUzer  should  be  as  low  as  possible,  and  it  must  never  be 
greater  than  the  resistance  of  any  of  the  leads  from  the  dynamos  to  the  bus  bar.  Sometimes  a 
third  wire  is  run  to  the  switchboard  from  each  dynamo  and  there  connected  to  an  eqalizer  bar, 
but  the  usual  practice  is  to  run  the  equalizer  directly  between  the  dynamos  and  to  place  the 
equalizer  switches  on  pedestals  near  the  machines.  This  shortens  the  connections  and  leads  to 
better  regulation.  The  positive  and  equalizer  switches  of  each  machine  differ  in  pressure  only 
by  the  slight  drop  in  the  series  coil,  and  in  some  large  stations  these  two  switches  are  placed 
side  by  side  on  a  pedestal  near  the  machine.  In  such  cases,  the  equalizer  and  positive  bus  bars 
tire  often  placed  under  the  floor  near  the  machines,  so  that  all  leads  may  be  as  short  as  possible. 
If  all  the  dynamos  be  of  equal  capacity,  all  the  leads  to  bus  bars  should  be  of  the  same  length, 
and  it  is  sometimes  necessary  to  loop  some  of  them. 


CIO  HAWKINS  ELECTRICITY 

If  the  difference  be  small,  it  may  be  compensated  by  means 
of  the  hand  regulator;  if  large,  however,  other  means  must  be 
taken  to  cause  the  machines  to  take  up  their  due  proportion  of 
the  load. 

If  the  series  coils  of  the  several  dynamos  be  pro\aded  with 
small  adjustable  resistances,  in  the  form  of  German  silver  or 
copper  ribbon  inserted  in  series  with  the  coils,  the  distribution 
of  the  current  in  the  latter  m.ay  be  altered  by  varying  the 
resistance  attached  to  the  individual  coils.  The  effect  of  the 
series  coils  upon  the  individual  armatures  in  raising  the  pressure 
may  be  adjusted,  and  the  load  thus  evenly  divided  among  the 
machines. 

Shunt    and    Compound   Dynamos    in   Parallel. — It  is 

not  practicable  to  run  a  compovmd  dynamo  and  a  shunt 
dynamo  in  parallel,  for,  unless  the  field  rheostat  of  the  shunt 
machine  be  adjusted  continually,  the  compound  dynamo  will 
take  more  than  its  share  of  the  load. 


DYNAMO  FAILS  TO  EXCITE  611 


CHAPTER   XXXI 
DYNAMO   FAILS   TO   EXCITE 


This  trouble  is  of  frequent  occurrence  in  both  old  and  new 
machines.  If  a  dynamo  fail  to  excite,  the  operator  should  first 
see  that  the  brushes  are  in  the  proper  position  and  making  good 
contact,  and  that  the  external  circuit  is  open  if  the  machine  be 
shunt  wound,  and  closed  if  series  wound. 

In  starting  a  dynamo  it  should  be  remembered  that  shunt  and 
compound  machines  require  an  appreciable  time  to  build  up, 
hence,  it  is  best  not  to  be  too  hasty  in  hunting  for  faults. 

The  principal  causes  which  prevent  a  dynamo  building  up  are : 

1.  Brushes  not  properly  adjusted; 

2.  Defective  contacts; 

3.  Incorrect  adjustment  of  regulators; 

4.  Speed  too  low; 

5.  Insufficient  residual  magnetism; 

6.  Open  circuits; 

7.  Short  circuits; 

a.  In  external  circuits; 

b.  In  dynamo. 

8.  Wrong  connections; 

9.  Reversed  field  magnetism. 

Brushes  not  Properly  Adjusted.— If  the  brushes  be  not  in 
or  near  their  correct  positions,  the  whole  of  the  voltage  of  the 
armature  will  not  be  utilized,  and  will  probably  be  insufficient  to 


612  HAWKINS  ELECTRICITY 

excite  the  machine.  If  in  doubt  as  to  the  correct  positions,  the 
brushes  should  be  rotated  by  means  of  the  rocker  into  various 
points  on  the  commutator,  sufficient  time  being  given  the  ma- 
chine to  excite  before  mo^•ing  them  into  a  new  position. 


Defective  Contacts. — If  the  different  points  of  contact  of 
the  connections  of  the  inachine  be  not  kept  thoroughly  clean  and 
free  from  oil,  etc.,  it  is  probable  that  enough  resistance  will  be 
interposed  in  the  path  of  the  exciting  current  to  prevent  the 
machine  building  or  exciting.  Each  of  the  contacts  should 
therefore,  be  examined,  cleaned,  and  screwed  up  tight. 

Oues.  Which  of  the  contacts  should  receive  special 
attention? 

Ans.  The  contact  faces  of  the  brushes  and  surface  of  the 
commutator.  These  are  very  frequently  covered  with  a  slimy 
coating  of  oil  and  dirt,  which  is  quite  sufficient  to  prevent  the 
machine  exciting. 

Incorrect  Adjustment  of  Regulators. — When  shimt  and 
compound  machines  are  provided  %\-ith  field  regtUators,  it  is 
possible  that  the  resistance  in  circuit  may  be  too  great  to  permit 
the  necessary  strength  of  exciting  current  passing  through  the 
field  Vi4ndings.  Accordingly,  the  fault  is  corrected  by  cutting 
out  more  or  less  of  the  resistance.  The  field  coils  of  series  ma- 
chines are  sometimes  pro\-ided  v\-ith  short  circuiting  switches  or 
resistances  arranged  to  shunt  the  current  across  the  field  coils. 
If  too  much  of  the  current  be  shunted  across,  the  switch  should 
be  opened,  or  if  there  be  a  regulator,  it  shovild  be  so  adjusted 
that  it  Vi-ill  pass  enough  current  through  the  field  windings  to 
excite  the  machine. 


DYNAMO  FAILS  TO  EXCITE 


613 


Speed  too  Low. — In  shunt  and  compound  dynamos  there  is 
a  certain  critical  speed  below  which  they  will  not  excite.  If  the 
nonnal  speed  of  the  machine  be  known,  it  can  be  seen  whether 
the  failure  to  excite  arises  from  this  cause,  by  measuring  the 
speed  of  the  armature  with  a  speed  indicator.  In  all  cases  it  is 
advisable,  if  the  machine  do  not  excite  in  the  course  of  a  few 
minutes,  to  slightly  increase  the  speed.  As  soon  as  the  voltage 
rises,  the  speed  may  be  reduced  to  its  regular  rate. 


4>0<K> 


Fig.  692. — Method  of  testing  for  break  by  short  circuiting  the  terminals  of  the  machine.  If 
the  external  circuit  test  out  apparently  all  right,  and  there  be  no  defective  contacts  in 
any  part  of  the  machine,  and  all  short  circuiting  switches,  etc.,  be  cut  out  of  circuit,  the 
machine  still  refusing  to  excite,  short  circuiting  the  terminals  of  the  machine  fhould  be  tried. 
This  should  be  done  very  cautiously,  especially  in  case  of  a  high  tension  machine.  It  is 
advisable  to  have,  if  possible,  only  a  portion  of  the  load  in  circuit,  and  the  short  circuit 
should  be  effected  as  shown  in  the  figure.  The  short  circuit  may  be  made  by  momentarily 
bridging  across  the  two  terminals  of  the  machine  with  a  single  piece  of  wire.  As  this,  how- 
ever, is  liable  to  burn  the  terminals,  a  better  plan  is  to  fix  a  short  piece  of  scrap  wire  in  one 
terminal,  and  then  with  another  piece  of  insulated  wire  make  momentary  contacts  with  the 
other  terminal  and  the  short  piece  of  wire.  If  the  machine  excite,  it  will  be  at  once  evident 
by  the  arc  which  occurs  between  the  two  pieces  of  wire.  As  the  voltage  of  a  series  machine 
when  induced  to  build  in  this  manner  generally  rises  very  rapidly,  great  care  should  be 
taken  that  the  contact  is  at  first  only  momentary,  merely  a  rubbing  or  scraping  touch 
of  the  wires.  The  contact  may  be  prolonged  if  the  machine  do  not  excite  at  the  first 
contact.  Compound  wound  machines  can  often  be  made  to  excite  quickly  by  short  cir- 
cuiting their  terminals  in  this  manner. 


614 


HAWKINS  ELECTRICITY 


Insufficient  Residual  Magnetism. — This  fault  is  not  of 
frequent  occurrence;  it  takes  place  chiefly  when  the  dynamo  is 
new,  and  may  be  remedied  by  passing  the  ciirrent  from  a  few 
storage  cells,  or  from  another  dynamo,  for  some  time  in  the 
proper  direction  through  the  field  coils.  If  a  heavy  current,  such 
as  is  obtainable  from  a  storage  battery,  be  not  available,  and  the 
machine  be  shunt  or  compound  wound,  a  few  primary  cells 
arranged  as  in  fig.  693  will  generally  sufl5ce. 


Pig.  693. — Method  of  overcoming  insufBcient  residual  magnetism.  The  flexible  "lead"  L  cf 
the  dynamo  p  is  disconnected  from  the  positive  terminal  of  the  machine,  and  is  connected 
to  the  negative  or  zinc  pole  of  the  battery  B.  the  other  or  positive  carbon  pole  being  con. 
nected  to  the  terminal,  from  which  the  lead  was  removed,  and  shunt  circuit  S.  As  thus; 
arranged,  it  will  be  seen  that  the  battery  B  is  in  series  with  the  armature  and  shunt  circuit, 
and  therefore  its  voltage  will  be  added  to  any  small  voltage  generated  in  the  armature. 
When  the  machine  is  started,  the  combined  voltages  will  probably  be  able  to  send  sufficient 
current  through  the  shunt  to  excite  the  machine.  As  the  voltage  rises  and  the  strength 
of  the  current  in  the  shunt  windings  increases,  the  flexible  lead  may  be  again  inserted  into 
the  terminal  from  which  it  was  removed.  The  battery  will  thus  be  short  circuited,  and 
may  be  cut  out  of  circuit  without  any  danger  of  breaking  the  shunt  circuit,  and  thus  caus- 
ing the  machine  to  demagnetize. 


Open  Circuits. — Dynamos  are  affected  by  open  circuits  in 
different  ways,  depending  upon  the  type.  Series  machines  re- 
quire closed  circuit  to  build  up,  while  an  open  circuit  is  necessary 


DYNAMO  FAILS  TO  EXCITE  G15 

with  the  shunt  machine.  An  open  circuit  may  be  due  to:  1, 
broken  wire  or  faulty  connection  in  the  machine;  2,  brushes 
not  in  contact  with  commutator;  3,  safety  fuse  blown  or  re- 
moved; 4,  circuit  breaker  open;  5,  switch  open;  6,  external 
circuit  open.  If  the  trouble  be  due  merely  to  the  switch  or 
external  circuit  being  open,  the  magnetism  of  a  shunt  machine 
may  be  at  full  strength,  and  the  machine  itself  may  be  working 
perfectly,  but  if  the  trouble  be  in  the  machine,  the  field  mag- 
netism will  probably  be  very  weak.  Open  circuits  are  most 
likely  to  occur  in: 

1.  The  armature  circuit; 

2.  The  field  circuit; 

3.  The  external  circuit. 

When  the  open  circuit  is  due  to  the  brushes  not  making  good  contact, 
simple  examination  generally  reveals  the  fact. 

Oues.    What  causes  breaks  in  the  field  circuit? 

Ans.  Bad  contacts  at  the  terminals,  broken  connections,  or 
fracture  of  the  coil  windings. 

Oues.    How  is  the  field  circuit  tested  for  breaks? 

Ans.  The  flexible  leads  attached  to  the  brushes  are  removed 
from  their  connections  with  the  field  circuit,  and  the  latter  is 
then  tested  for  conductivity  with  a  galvanometer. 

Oues.  Where  is  a  break  likely  to  occur  in  a  shunt 
machine? 

Ans.  In  the  hand  regulator  through  a  broken  resistance  coil 
or  bad  contact. 

Very  frequently  the  fault  occurs  in  the  connecting  wires  leading  from 
the  machine  to  the  hand  regulator  fixed  upon  the  switchboard,  or  in 
the  short  wires  connecting  the  field  coils  to  the  terminals  or  brushes. 


616 


HA  WKIXS  ELECTRICITY 


The  insulation  of  a  broken  wire  will  sometimes  hold  the  two  ends 
together  30  as  to  defy  any  but  the  most  careful  inspection  or  exam- 
ination; therefore,  in  order  to  avoid  loss  of  time,  it  is  ad\Tsable  to  dis- 
connect the  wires  if  possible,  and  test  each  separately  for  conducri\-ity 
■wnth  a  batten,'  and  galvanometer  connected,  as  in  fig.  694.  If  the  fault 
be  not  located  in  the  various  connections,  the  magnet  coils  should  be 
tested  with  the  batten.-  and  galvanometer  coupled  up  as  in  fig.  706,  care 
being  first  taken  to  disconnect  the  ends  of  each  of  the  coils.  A  faiilt)- 
coil  will  not  show  any  deflection  of  the  gal\'anometer. 


Fig.  694. — Method  of  testing  dj-namo  for  short  circuits.  la  the  figure,  ooe  pole  of  the  battel^ 
B  is  placed  in  contact  with  the  frame  of  the  xn.aciui:e  at  a  point  wliicb  Ins  ptevioady  been 
well  scraped  and  cleaned;  the  other  pole  is  connected  to  one  of  the  galvanometer  temnnals 
as  shown.  The  other  terminal  of  the  gal%-anomeier  is  connected  to  eadi  of  the  dynamo 
terminals  T  T  under  test  in  turn.  If  a  deflection  of  the  needle  be  prodnoed  when  the  gal- 
vanometer terminal  is  in  contact  with  either,  the  terminals  are  in  omtact  with  the  frame. 
and  they  should  then  be  removed,  and  the  fault  repaired  by  additional  insalataoo  or  by 
rednsulating. 


Oues.     At  what  point  of  a  shunt  coil  does  a  break  usually 
occur? 

Ans.     At  the  point  where  the  wire  passes  through  the  flanges 
of  the  spool  or  bobbin. 


DYNAMO  FAILS  TO  EXCITE  617 

Oues.     How  should  the  coil  be  repaired  ? 

Ans.  In  most  cases  a  little  of  the  wood  or  metal  of  which  the 
flanj^je  is  made  can  be  gouged  or  chipped  out,  and  a  new  connect- 
ing wire  soldered  on  to  the  broken  end  of  the  coil  without  much 
difficulty. 

If  it  be  necessary  to  take  the  magnets  apart  at  any  time,  care  should 
be  taken  in  putting  them  together  again  to  wipe  all  faces  perfectly  clean, 
and  screw  up  firmly  into  contact,  and  to  see  that  the  connections  of  the 
coils  are  made  as  they  were  before  being  taken  apart. 


Fig.  695. — Watson  armature  discs.     Each  laminaiion  is  made  from  low  carbon  electrical  steel 
of  high  magnetic  permeability.     Each  disc  is  annealed  and  afterwards  varnished. 

If  the  faulty  coil  cannot  be  repaired  quickly,  and  the  machine  is 
urgently  required,  the  coil  may  be  cut  out  of  circuit  entirely,  or  short 
circuited,  and  the  remaining  coils  coupled  up  so  as  to  produce  the 
correct  polarity  in  the  pole  pieces. 

Oues.  What  trouble  is  liable  to  be  encountered  in 
operating  after  cutting  out  a  coil? 

Ans.  The  remaining  coils  are  liable  to  heat  up  to  a  greater 
extent  than  formerly,  owing  to  the  increased  current,  hence  it 
is  advisable  to  proceed  cautiously  in  starting  the  dynamo,  since 
the  temperature  may  exceed  a  safe  limit.  If  this  occur,  a  resist- 
ance may  be  put  in  circuit  with  the  field  coils,  or  the  speed  of 
the  dynamo  reduced. 


618 


HAWKINS  ELECTRICITY 


Fig.  696.— Fort  Wayne 
pedestal  typ)e  commu- 
tator truing  device. 
When  this  device  is 
used,  the  armature  is 
revolved  in  its  own 
bearings  by  means  of 
a  handle  clamped  to 
the  pulley.  The  tool 
has  a  horizontal  travel 
of  21  ins.,  (being  3  ins, 
wide  inside  the  fasten- 
ing bolt  in  the  base), 
and  a  vertical  adjust- 
ment of  12  ins.,  adapt- 
ing it  to  machines  with 
commutators  up  to  36 
ins.  in  diameter. 


Fig.  697.— Fort  Wayns 
yoke  type  commutator 
truing  device  for  ma- 
chines ha\-ing  brush 
mechanism  mounted 
on  a  yoke  carried  by 
the  field  frame.  It 
consists  of  a  carriage 
for  the  tool  holder 
ha\'ing  a  screw  feed 
and  a  bracket  for  at- 
taching to  the  brush 
yoke.  The  bracket  re- 
places two  brush  holder 
brackets  on  the  brush 
yoke ,  and  is  made  to  fit 
the  yoke  of  the  par- 
ticular machine  on 
which  it  is  to  be  used. 


DYNAMO  FAILS  TO  EXCITE  619 

Oues.  What  kind  of  dynamo  is  affected  by  breaks  in 
the  external  circuit? 

Ans.     A  scries  dynamo. 

Oues.     Name  the  kind  of  break  that  is  difficult  to  locate. 

Ans.     A  partial  break. 

Short  Circuits. — In  a  series  or  compound  dynamo  a  short 
circuit  or  heavy  load  will  overload  the  machine  and  cause  the 
fuses  to  blow.  A  shunt  machine  will  not  excite  under  these 
circumstances,  for  the  reason  that  practically  the  whole  of  the 
current  generated  in  the  armature  passes  direct  to  the  external 
circuit,  and  the  difference  of  potential  between  the  shunt  ter- 
minals is  practically  nil. 

Oues.  What  should  be  done  if  it  be  suspected  that  the 
failure  to  excite  arises  from  this  cause? 

Ans.  The  main  leads  should  be  taken  out  of  the  dynamo 
terminals,  then,  if  due  to  this  cause,  the  machine  will  excite. 

Oues.  What  parts  of  a  dynamo  are  specially  liable  to 
be  short  circuited  ? 

Ans.  The  terminals,  brush  holders,  commutator,  armature 
coils  and  field  coils. 

Oues.  How  are  the  terminals  liable  to  be  short  cir- 
cuited ? 

Ans.  The  terminals  of  the  various  circuits  of  the  machine 
are  liable  to  be  short  circuited,  either  through  metallic  dust 
bridging  across  the  insulation,  or  through  the  terminals  making 
direct  contact  with  the  frame  of  the  machine. 

The  various  terminals  should  be  examined,  and  if  the  fault  cannot  be 
located  by  inspection,  they  should  each  be  disconnected  from  their  cir- 
cuits and  tested  with  a  battery  and  galvanometer  arranged  as  in  fig.  694, 


(^20 


HAWKIXS  ELECTRICITY 


Oues.  What  precaution  should  be  taken  with  the 
brush  holders  .- 

Ans.  Since,  they  are  liable  to  be  short  circuited  through  the 
rocker  by  metallic  dust  lodging  in  the  insulating  washers,  they 

should  be  kept  clean. 

Oues.    How  are  the  brush  holders  tested  ? 
Ans.     A  galvanometer  and  battery  are  connected  in  series 
with  one  terminal  of  the  galvanometer  connected  to  one  set  of 

SUPP&  MAGNTIZING  COIL 


COIL  UNDER  TEST 


LAMINATED 
IRON  GORE 


Pig.  69&. — Field  coil  testing  with  telephone  rectiver.  In  the  method  here  shown,  a  telephone 
receiver  is  connected  in  series  with  two  s>iimietrically  placed  coils  A  and  B.  Ver>-  little 
sound  will  be  heard  when  the  flux  through  the  two  coils  AB  is  the  same;  but  if  a  short- 
circmted  coil  is  being  tested,  the  fluxes  through  the  coils  A,  B  will  not  t>e  equal  and  a  noise 
can  be  heard  in  the  leceivier. 

brushes;  the  imconnected  terminal  of  the  battery  is  then  con- 
nected with  the  other  set  of  brushes.  A  deflection  of  the  needle 
will  indicate  a  short  circviit. 

Oues.     What  is  the  effect  of  a  short  circuit  in  the  field 
coils  or  field  circuit? 

Ans.     The  machine  generally  refuses  to  e.xcite. 
Oues.     How  are  the  field  coils  tested  for  short  circuit  ? 
Ans.     By  measuring  the  resistance  of  each  coil  ■with  an  ohmmeter 
or  Wheatstone  bridge.     The  faulty  coils  will  show  a  much  less 


DYNAMO  FAILS  TO  EXCITE  621 

resistance  than  the  perfect  coils.  The  fault  may  also  be  dis- 
covered and  located  by  passing  a  strong  current  from  a  battery  or 
another  dynamo  through  each  of  the  coils  in  turn,  and  observing 
the  relative  magnetic  effects  produced  by  each  upon  a  bar  of  iron 
held  in  their  vicinity. 

The  short  circuit  may  be  in  the  terminals  or  connections,  and  these 
should  first  be  examined  and  tested. 

Some  series  dynamos  are  provided  with  a  resistance,  arranged  in 
parallel  or  shunt  with  the  field  coils,  to  divert  a  portion  of  the  current 
therefrom,  and  thus  regulate  the  output. 


Pig.  699. — Watson  armature  complete.  The  armature  coils  are  form  wound,  heavily  insulated 
and  so  mounted  on  the  core  as  to  insure  rapid  dissipation  of  heat  by  ventilation.  Each 
coil  is  protected  by  an  insulating  sheath  and  tape  covering  before  mounting.  The  arma- 
ture is  baked  after  the  coils  are  mounted  to  drive  out  all  moisture,  then,  while  hot,  is 
treated  with  insulating  compound  and  again  baked  twelve  hours. 

When  making  a  series  dynamo  excite,  all  resistances  and  controlling 
devices  should  be  temporarily  cut  out  of  circuit  by  opening  the  shunt 
circuit.  Series  machines  have  frequently  a  switch  which  short  circuits 
the  field  coils.  Care  should  be  taken  that  this  is  open,  or  otherwise  the 
machine  will  not  excite. 

Wrong  Connections. — When  a  machine  is  first  erected,  the 
failure  to  build  up  may  be  due  to  incorrect  connections. 
The  whole  of  these  latter  should  therefore  be  traced  or  followed 


622  HAWKINS  ELECTRICITY 

out,  and  compared  with  the  diagrams  of  dynamo  connections 
given  in  figs.  190  to  198. 

Sometimes  errors  are  made  in  connecting  the  field  coils,  causing  them 
to  act  in  opposition.  This  may  occur  when  the  dynamo  is  a  new  one  or 
the  coils  have  been  removed  for  repairs.  It  may  be  caused  either  through 
the  coils  having  been  put  on  the  field  cores  the  wrong  way,  or  through 
incorrect  coupling  up.  Under  these  circumstances,  the  dynamo,  if  bi- 
polar, will  fail  to  excite;  and  if  multipolar,  poles  will  be  produced  in  the 
yokes,  etc.  It  may  be  remedied  by  removing  one  of  the  coils  from  the 
core  and  putting  it  on  the  reverse  way,  or  by  reversing  its  connections. 
The  correctness  of  connections  of  all  the  coils  should  be  verifisd. 

In  compound  dynamos  it  sometimes  happens  that  the  machine  will 
excite  properly,  but  that  the  series  coils  tend  to  reverse  the  polarity  of 
the  dynamo,  thus  reducing  the  voltage  as  the  load  upon  the  machine 
increases.  This  may  be  detected  when  the  machine  is  loaded  by  short 
circuiting  the  series  coils,  not  the  terminals.  If  the  voltage  rise  in  doing 
this,  the  series  coils  are  acting  in  opposition  to  the  shunt  coils,  and  the 
connections  of  the  series  coils  must  be  reversed. 


Reversed  Field  Magnetism. — This  is  sometimes  caused 
by  the  nearness  of  other  dynamos,  but  is  generally  due  to  re- 
versed connections  of  the  field  coils.  Under  such  conditions  the 
field  coils  tend  to  produce  a  polarity  opposed  to  the  mag- 
netization to  which  they  owe  their  current,  and  therefore  the 
machine  will  refuse  to  excite  until  the  field  connections  arc 
reversed,  or  a  current  is  sent  from  another  dynamo  or  a  battery 
through  the  field  coils  in  a  direction  to  produce  the  correct 
polarity  in  the  pole  pieces. 


ARMATURE  TROUBLES  623 


CHAPTER  XXXII 
ARMATURE   TROUBLES 


A  large  proportion  of  the  mishaps  and  breakdowns  which 
occur  with  dynamos  and  motors  arise  from  causes  more  strictly 
wdthin  the  province  of  the  man  in  charge  than  in  that  of  the 
designer.  The  armature,  being  a  complex  and  delicately  built 
structure,  is  subject  in  operation  to  various  detrimental  in- 
fluences gi\ing  rise  to  faults. 

IVIany  of  the  faults  which  occur  are  avoided  by  operators 
better  informed  as  to  the  electric  and  magnetic  conditions  which 
obtain  in  the  running  of  the  machine,  especially  the  mechanical 
stresses  on  the  copper  inductors  due  to  the  magnetic  field  and 
the  necessity  of  preserving  proper  insulation. 

The  chief  mishaps  to  which  armatures  are  subject  are  as 
follows : 

1.  Short  circuits; 

a.  In  individual  coils; 

b.  Between  adjacent  coils; 

c.  Through  frame  or  core; 

d.  Between  sections  of  armature; 

e.  Partial  short  circuits. 

2.  Grounds; 

3.  Breaks  in  armature  circuit. 


G24 


HAWKIXS  ELECTRICITY 


Short  Circuit  in  Individual  Coils. — This  is  a  common 
fault,  which  makes  its  presence  known  by  a  \-iolent  heating  of 
the  armature,  flashing  at  the  commutator,  flickering  of  the  light 
on  hghting  circuits,  and  by  a  smell  of  burning  varnish  or  over- 
heated insulation.  When  these  indications  are  present,  the 
machine  should  be  stopped  at  once,  otherwise  the  armature  is 
Uable  to  be  burnt  out.  The  fault  is  due  either  to  metallic  dust 
lodging  in  the  insulation  between  adjacent  bars  of  the  com- 


FiG.  700. — Method  of  lofaringsh<»tcircaitedannatnrecwL  Disooooect  the  external  and  field 
circnits  from  the  annatore.and  pass  a  laige  cnneat — say  bom  20  to  100  amperes — from  a 
battery  (B>  or  srnnttwr-  dynamo  throng  the  wh<^  aimatme  by  means  of  the  bntdies. 
Then,  having  prevkmsty  well  cleaned  the  oommntatw.  measure  the  difierenoe  of  potential 
between  adjacent  segments  all  round  the  commutator  (C),  by  means  oS  a  voltmeter  or 
galvanometer  (O;.  the  terminals  at  which  are  connected  to  adjacent  segments,  as  shown. 
The  short  arcnited  coQ  or  coils  will  be  located  by  the  difference  of  potential  between  the 
oorre^mnding  segments  being  little  or  nothing.  It  may  be  remarked,  however,  that  this 
is  not  always  a  decisive  test.  In  some  cases  the  short  circait  may  be  intermittent,  or  may 
disappear  as  soon  as  the  armatnre  ceases  to  rotate.  In  sach  cases,  the  short  arcait  is  caused 
by  die  wire  coming  into  contact  throo^  the  action  of  the  centrifugal  forces  developed  by 
the  rotation  of  the  armature. 


mutator,  or  to  one  or  more  convolutions  of  the  coils  coming  into 
contact  with  each  other,  either  through  a  metaUic  filing  be- 
coming embedded  in  the  insulation  or  damage  to  the  insulation. 

Oues.     How  is  the  fault}'  coil  located? 

Ans.  When  the  machine  is  stopped,  the  faulty  coil,  if  not 
burnt  out,  can  generally  be  located  by  the  baked  appearance  of 
the  varnish  or  insulation,  and  by  its  excessive  temperature  over 


ARMATURE  TROUBLES 


625 


the  rest  of  the  coils,  being  detected  also  by  the  baked  appear- 
ance of  the  "^^arnish  or  insulation. 

Oues.  What  should  be  done  if  the  machine  do  not 
build,  and  it  be  suspected  that  the  fault  is  due  to  short 
circuited  armature  coils? 

Ans.  The  field  magnets  should  be  excited  by  the  current 
from  a  storage  battery  or  another  dynamo,  and,  having  raised  the 


Fig.  701. — Test  for  break  in  armature  lead.  Clean  the  brushes  and  commutator,  and  apply 
current  from  a  few  cells  of  battery  having  a  telephone  receiver  in  circuit  as  shown  in  the 
figure.  If  the  machine  have  more  than  two  brushes,  connect  the  leads  to  two  adjoining 
brushes  and  raise  the  others.  IS'ow  rotate  the  armature  slowly  by  hand  and  there  will 
be  a  distinct  click  in  the  receiver  as  each  segment  passes  under  the  brushes  until  one  brush 
bears  on  the  segment  at  fault,  when  the  clicking  will  cease.  In  making  this  test,  the 
brushes  must  not  cover  more  than  a  single  segment. 


brushes  from  contact  with  the  commutator,  the  armature  should 
be  run  for  a  short  time.  In  stopping,  the  faulty  coil  or  coils  may 
be  located  by  the  heat  generated  by  the  short  circuit. 

When  the  dynamo  is  started  for  the  purpose  of  localizing  a  short 
circuit,  precautions  should  be  taken,  and  the  machine  only  run  for  a  few 
minutes  at  a  time  until  the  faulty  coil  is  detected. 

When  the  faulty  coil  has  been  located,  the  insulation  between  the 
segments  of  the  commutator  to  which  its  ends  are  connected  should  be 
carefully  examined  for  anything  that  may  bridge  across  from  segment 
to  segment,  and  scraped  clean.    If  the  commutator  be  apparently  all 


626 


HAWKINS  ELECTRICITY 


right,  the  fault  probably  lies  in  the  winding.  The  insulation  of  the 
winding  should  be  carefully  examined,  and  any  metallic  filings  or  other 
particles  discovered  therein  carefully  removed,  and  a  little  shellac  var- 
nish applied  to  the  faulty  part. 

Oues.  If  the  insulation  on  adjacent  conductors  has 
been  abraded,  how  should  it  be  repaired? 

Ans.  A  small  boxwood  or  other  hardwood  wedge,  coated  with 
shellac  varnish  should  be  driven  in  tightly  between  the  wire, 
this  will  generally  be  sufficient. 


Fig.  702. — ^Bar  tofaartest  foropen  drcoitin  omlor  short  cucmtin  tne  ocal  or  between  segments. 
If.  in  testing  as  in  fig.  701  .<m  rotating  the  armatore  oomid^ely  aroond.  the  leoeiver  indicate 
no  break  in  the  leads,  cxnnect  the  battery  leads  directly  to  the  bm^ies.  as  shown  in  the 
above  figure,  and  tooch  the  connections  frcMn  the  receiver  to  two  adjacent  bars,  woridng 
from  bar  to  bar.  The  cHcIdng  should  be  substantaally  the  same  between  any  two  oom- 
nmtator  bars;  if  the  clicking  suddenly  rise  in  tone  between  two  bars,  it  indicates  a  high 
resistance  in  the  coQ  or  a  break  (open  dicuit). 


Oues.  If  a  faultj'  coil  cannot  be  quickly  repaired  and 
the  dynamo  be  needed,  what  should  be  done? 

Ans.  The  coil  may  be  cut  out  of  circuit,  and  the  correspond- 
ing commutator  segments  connected  together  with  a  piece  of 
wire  (of  a  size  proportionate  to  the  amount  of  current  to  be 
carried),  soldered  to  each.  It  will  not  l^e  necessary'  to  cut  out 
and  remove  the  entire  coil. 


ARMATURE  TROUBLES 


627 


If  the  active  portions  only  be  separated  so  that  they  do  not  form  a 
closed  circuit,  it  will  answer  the  purpose.  If  the  wires  be  cut  with  a  chisel 
at  the  point  where  they  pass  over  the  ends  of  the  core,  and  the  ends 
separated,  it  will  be  quite  as  eflFective  as  removing  the  entire  coil.  It  is 
wise,  of  course,  to  rewind  tlie  coil  at  the  first  opportunity. 

Short  Circuits  between  Adjacent  Coils. — In  ring  arma- 
titres  the  presence  of  this  fault  does  not  necessarily  imply  that 
the  machine  will  not  build;  in  drum  armatures,  wound  into  a 
single  layer  of  conductors',  it  entirel}^  prevents  this  occurring. 


Fig.  703. — Alternate  bar  test  for  short  circuit  between  sections.  Where  two  adjacent  com- 
mutator bars  are  in  contact,  or  a  coil  between  two  segments  becomes  short  circuited,  the 
bar  to  bar  test  described  in  fig.  702  will  detect  the  fault  by  the  telephone  receiver  remaining 
silent.  If  a  short  circuit  be  found,  the  leads  from  the  receiver  should  then  include  or 
straddle  three  commutator  bars,  as  here  shown.  The  normal  click  will  then  be  twici  that 
between  two  segments  until  the  faulty  coils  are  reached,  when  the  clicking  will  be  less. 
When  this  happens,  test  each  coil  for  trouble  and.  if  indi\'idually  they  be  all  right ,  the  trouble 
is  between  the  two.  To  test  for  a  ground  place  one  terminal  of  the  receiver  on  the  shaft 
or  frame  of  the  machine,  and  the  other  on  the  commutator.  If  there  be  a  click  it  indicates 
a  ground.  Move  the  terminal  about  the  commutator  until  the  least  clicking  is  heard  and 
at  or  near  that  point  will  be  found  the  contact.  Grounds  in  field  coils  can  be  located  in 
the  same  manner. 

Reference  to  a  winding  diagram  will  show  that  adjacent  coils 
are  during  a  certain  period  of  the  revolution  at  the  full  difference 
of  pressure  generated  by  the  machine.  Hence,  if  any  two  adjacent 
coils  be  connected  together  or  short  circuited,  the  whole  of  the 
armature  will  be  practically  closed  on  itself,  any  current  gener- 
ated flowing  within  the  armature  only. 


628 


HAWKINS  ELECTRICITY 


Large  drum  armatures  wound  with  compressed  and  stranded 
bars  and  connectors  are  particularly  susceptible  to  this  fault,  a 
slight  blow  generally  forcing  one  or  more  of  the  strands  into 
contact  •v\-ith  the  adjacent  bars,  thus  short  circuiting  the  arma- 
ture, and  rendering  it  practically  useless  so  far  as  the  generation 
of  current  is  concerned.  In  this  class  of  short  circuit  in  drum 
armatures,  the  method  of  locating  the  faulty  coils  by  exciting 
the  field,  and  running  the  armatures  on  open  circuit,  does 
not  apply,  for  the  reason  that  the  whole  armature  will  be  heated 
equally. 


Pig.  704. — Method  of  locating  short  circuits  between  adiacent  armature  coils.  F::.:'.  •.  -  -^  ~ -ti- 
key  wrench  to  the  rim  of  the  pulley,  or  a  crank  to  the  shaft.  Now.  excite  the  f.tli?,  ^nc. 
to  make  the  effects  more  marked,  connect  the  coils  in  parallel.  When  this  has  bwn  dec 
it  will  require  considerable  force  to  rotate  the  armature,  and  then  it  will  move  qxL.: 
slowly,  except  at  one  position.  When  this  position  has  been  found,  mark  the  armatii:-. 
at  points  in  the  center  of  the  pole  pieces  at  points  AandB  andat  both  ends  of  the  armature. 
The  explanation  is  that  both  halves  of  the  armature  oppose  one  another  at  this  position; 
but  when  not  at  these  points  a  continuous  circuit  is  formed,  and  the  resultant  magnetic 
effect  is  considerable.  The  "cross"  or  "short "  circuit  is  nearly  always  found  on  the  com- 
mutator end  in  the  last  half  of  the  windinR.  where  the  wires  pass  down  througrh  the  first 
half  terminals.  This  apphes  to  an  unequal  winding.  In  armatures  where  the  windings  are 
equal,  it  is  as  liable  to  occur  at  one  point  as  at  another.  With  this  method  a  defect  can  be 
found  and  remedied  in  a  few  moments,  for  it  has  always  been  a  simple  matter  to  repair  it 
when  discovered.  These  results  can  be  observed  in  a  perfect  armature  by  connecting  the 
opposite  sections  of  the  commutator. 


A  method  of  locating  such  fault  is  illustrat£>d  in  fig.  704.  This  applies 
to  drum  wound  armatures.  Faialts  of  this  description  can  frequently 
be  discovered  by  a  carefiJ  inspection  of  the  windings  of  the  armature 
without  recourse  to  testing.  When  located,  the  faialt  can  usually  be  re- 
paired with  a  hardwood  wedge,  as  explained  above,  or  a  piece  of  mica  or 
vulcanized  fibre  cemented  in  place  with  shellac  \'amish. 


ARMATURE  TROUBLES 


629 


Short  Circuits  between  Sections  through  Frame  or  Core 

of  Armature.^Detection  of  this  fault  can  be  effected  by  the 
methods  described  above,  and  by  disconnecting  the  whole  of 
the  armature  coils  from  the  commutator  and  from  each  other, 
and  testing  each  separately  with  a  battery  and  galvanometer 
coupled  up  as  in  fig.  705,  one  mre  being  connected  to  the  shaft 
and  the  other  to  the  end  of  the  coil  under  test.  As  a  rule,  there 
is  no  way  of  remed^Hing  this  fault  other  than  unwinding  the 
defective  coils,  reinsulating  the  core,  and  rewinding  new  coils. 


Fig.  705. — Method  of  locating  short  circuits  between  coils  through  armature  core.  The 
galvanometer,  battery  and  coil  to  be  tested  are  connected  in  series  as  shown,  and  then  the 
unconnected  terminal  of  the  galvanometer  is  brought  into  contact  with  the  shaft.  If  then 
some  portion  of  the  insulation  of  the  wire  has  been  abraded  or  destroyed,  thus  bringing 
the  bare  wire  into  contact  with  the  metal  core,  as  at  A  in  the  figure,  the  needle  of  the  gal- 
vanometer will  be  deflected  since  a  closed  circuit  is  formed  through  the  core  and  wire.  If 
the  insulation  be  perfect,  the  needle  will  not  be  deflected.  It  will  thus  be  seen  that  in  the 
conductivity  test  (fig.  700)  it  is  necessary  that  the  needle  should  be  deflected,  or  turned, 
to  prove  that  all  is  right,  while  in  the  insulation  test  the  converse  holds  good;  if  the  needle 
be  deflected,  it  proves  that  the  insulation  is  broken  down. 


Short  Circuits  between  Sections  through  Binding  Wires. 

— This  fault  is  the  result  of  a  loose  winding,  and  is  caused  by  the 
insulation  upon  which  the  binding  wires  are  wound  giving  way, 
thus  bringing  coils  at  different  pressures  together.  As  a  conse- 
quence of  the  hea\'y  current  which  flows,  tne  binding  wires  are 
as  a  rule  imsoldered  or  burned.     The  location  of  the  fault  can 


630 


HAWKINS  ELECTRICITY 


therefore  be  effected  b}'  simple  inspection.  To  remedy,  it  will  be 
necessary  to  unwind  and  rewind  on  new  binding  wires,  on  bands 
of  mica  or  vulcanized  fibre,  soldering  at  intervals  to  obviate 
fl^-ing  asunder. 

Partial  Short  Circuits  in  Armatures. — This  is  usually  due 
to  the  presence  of  moisture  in  the  windings.  To  remedy  the 
fault,  the  armature  should  be  taken  out  and  exposed  to  a 
moderate  heat,  or  subjected  to  a  current  equal  to  that  ordina- 


FiG.  706. — Method  of  testing  for  breaks.  The  instruments  are  connected  as  shown.  B  is  the 
battery,  G  the  galvanometer,  ard  S  the  coil  of  wire  being  tested.  One  terminal  of  the 
battery  is  connected  to  a  terminal  of  the  galvanometer,  and  the  other  to  one  of  the  ends  of 
the  coil  under  test.  The  other  terminal  of  the  galvanometer  is  connected  to  the  other  end 
of  the  coil,  If  the  connecting  wires  be  making  good  electrical  contact  with  the  respective 
terminals,  and  the  wire  of  coil  being  tested  be  unbroken,  the  needle  of  the  galvanometer 
will  be  deflected  as  soon  as  a  closed  circuit  is  made  by  the  end  of  the  coil  coming  into  con- 
tact with  the  galvanometer  terminal.  If  the  wire  of  the  coil  be  broken  in  some  part  or  the 
ends  of  the  connecting  wires  do  not  make  good  electrical  contact  with  the  tenninals.  the 
needle  will  not  be  deflected.  In  order  to  prevent  mistakes,  it  is  ad\'isable  to  test  the  battery 
and  galvanometer  corihections  and  contacts  by  short  circuiting  or  bringing  the  ends  of  the 
wire  connecting  the  terminal  of  the  galvanometer  and  negative  pole  or  the  battery  together 
before  starting  to  test  the  circuit  or  coil.  If  the  needle  be  deflected,  the  connections  are 
all  right;  if  not  deflected,  there  is  a  bad  contact  somewhere,  which  must  be  made  good 
before  the  test  can  proceed. 


rily  given  by  the  dynamo.  Under  the  action  of  heat  or  of 
this  current  the  moisture  will  be  gradually  dispersed.  When 
thoroughly  dry,  and  while  still  warm,  a  coat  of  shellac  should 
be  applied  to  the  whole  of  the  windings. 


ARMATURE  TROUBLES 


631 


Burning  of  Armature  Coils. — The  reason  for  the  burning 
of  an  armature  coil  may  be  explained  as  follows:  The  coil, 
segments,  and  the  short  circuit  between  the  segments  form  a 
closed  circuit  of  low  resistance  so  that  it  is  only  necessary  to  have 
a  low  pressure  set  up  in  the  active  portion  of  the  coil  to  force  a 
very  large  current  through  the  coil  and  the  short  circuited  com- 
mutator bars.  The  heating  effect  of  this  current  is  sufficient  to 
burn  otit  the  coil. 


"IG.  707. — Watson  field  coils.  Automatic  machinery  is  employed  to  wind  these  coils;  after 
winding,  they  are  bound  with  tape,  then  baked  to  expel  all  moisture,  and  while  hot,  are 
saturated  with  an  insulating  compound  and  again  baked  for  twelve  hours  to  make  them 
practically  oil  and  water  proof.  Heavy  flexible:  eads  are  brought  out  to  avoid  danger  of 
breaking  or  other  damage. 


Cutting  Out  Damaged  Armature  Coils. — To  cut  out  a 

damaged  coil  from  an  armature,  first,  disconnect  the  coil  from 
the  commutator,  and  after  cutting  off  the  leads,  insulate  the 
exposed  parts  with  tape.  Then  connect  the  commutator  bars 
(which  were  connected  with  the  leads)  with  a  wire  of  the  same  size 
as  the  wire  winding. 

To  remove  the  coil  entirely,  cut  the  band  wire  or  remove  the 
wedges,  and  lift  up  a  sufficient  number  of  leads  and  coils  to 
pewiait  of  the  removal  of  the  damaged  coil. 


632 


HAWKINS  ELECTRICITY 


Grounds  in  Armatures. — These  faults  occur  when  the 
armature  coils  become  connected  to  the  frame  or  core  of  the 
armature.  When  this  grounding  is  confined  to  a  single  coil,  it 
is  not  in  itself  liable  to  do  damage.  A  simple  method  of  locating 
a  grounded  coil  is  illustrated  in  fig.  708. 


Fig.  708. — Method  of  locating  grounded  armature  coil.  B  is  a  batterj'  or  djTiamo  circuit 
giving  a  current  of  a  few  amperes  through  the  armature  by  its  ovm  brushes  (1  and  2).  At 
G  is  placed  a  roughly  made  galvanometer,  to  carry  some  25  amperes  or  so,  one  terminal 
being  in  connection  with  the  shaft  of  the  armature,  and  the  other  attached  to  a  movable 
brush  3.  Since  the  function  of  the  particular  galvanometer  is  simply  to  show  a  deflection 
when  a  current  is  passing,  and  to  mark  zero  when  there  is  none,  a  coil  of  thick  wire  with  a 
pocket  compass  in  the  center  wrill  do  all  that  is  required,  but  care  must  be  taken  to  remove 
it  sufficiently  far  away  from  the  disturbing  effects  of  the  armature  magnetism.  The  manner 
of  testing  is  as  follows:  Assume  a  steady  current  to  be  flowing  from  battery-  B  through  the 
armature;  touch  the  commutator  with  brush  3.  and  a  current  will  flow  through  G. 
Slowly  rotate  the  armature  or  the  brush  3  until  the  galvanometer  G  shows  no  deflection. 
The  coil  in  contact  with  3  will  be  found  to  be  grounded.  A  hand  regulator  or  rheostat  R 
may  be  inserted  in  series  with  the  battery  or  dynamo  circuit  to  regulate  the  strength  of  the 
current  passing. 

Oues.     What  is  the  advantage  of  this  test? 

Ans.  The  damaged  coil  can  be  located  \\'ithout  unsoldering 
the  coils  from  the  commutator,  which  is  sometimes  a  difficult 
operation  -without  proper  tools;  further,  the  fault  can  frequently 
be  repaired  VN-ithout  disconnecting  any  of  the  wires  if  its  exact 
position  be  determined. 


ARMATURE  TROUBLES 


633 


Magneto  Test  for  Grounded  Armatures. — A  magneto  test 
for  grounded  armatures  is  not  to  be  recommended,  as  armatures 
often  possess  sufficient  static  capacity  to  cause  a  magneto  to 
ring  even  though  there  be  no  leak.  This  is  due  to  the  alternating 
current  given  by  the  magneto  for  when  the  circuit  has  capacity 
it  acts  as  a  condenser  and  at  each  revolution  of  the  armature  of 
the  magneto  a  rush  of  current  goes  out  and  returns,  charging  the 
surfaces  of  the  conductor  alternately  in  opposite  directions,  and 
ringing  the  bell  during  the  process. 


Fig.  709. — Method  of  binding  armature  winding.  Complete  appliances  for  handling  annatures 
in  making  repairs  are  usually  not  available  with  most  street  railway  companies,  since  they  are 
so  seldom  required.  When  needed,  therefore,  some  temporary  contrivance  must  be  resorted 
to  for  help  in  the  dilemma.  Should  an  armature  burn  out,  some  local  concern  that  makes 
coils  and  rewinds  armatures  may  be  available  to  do  the  work;  again,  it  will  be  necessary 
to  send  to  the  manufacturers  for  a  man,  as  soon  as  coils  can  be  made  ready  for  the  work. 
In  no  case  should  any  but  an  experienced  man  be  given  charge  of  this  work.  But  if  there  be 
any  doubt  as  to  whether  the  armature  is  really  burnt  out,  let  a  competent  man  be  the 
judge.  When  a  large  armature  needs  repairing,  a  pair  of  chain  tongs  can  be  used  on 
some  part  of  the  shaft  when  putting  in  the  coils,  and  a  block  and  tackle,  as  shown,  can  be 
used,  when  putting  on  the  band  wires.  Do  not  finish  one  band  and  then  cut  off  the  wire, 
but  run  it  over  for  the  next,  etc.     Then  solder  and  trim  off  the  wires. 


Breaks  in  Armature  Circuit. — A  partial  or  complete  break 
in  the  armature  circuit  is  always  accompanied  by  heavy  sparking 
at  the  commutator,  but  not,  as  a  rule,  by  an  excessive  heating 
of  the  armature  or  slipping  of  the  belt,  and  this  enables  the 
fault  to  be  distinguished  from  a  short  circuit.  The  faulty  part 
can  always  be  readily  located  by  the  "  flat  "  which  it  produces 


634  HAWKINS  ELECTRICITY 

upon  the  surface  of  the  commutator.  The  armature  circuit 
being  open  at  the  faulty  part,  heavy  sparking  results  at  every 
half  revolution  as  the  brushes  pass  over  it,  and  as  a  consequence 
the  corresponding  segments  become  "  pitted  "  or  "  flattened  " 
with  respect  to  the  others;  they  may  easily  be  discovered  on 
examination. 

Breaks  in  the  armature  circuit  may  occur  in  either  the  commutator 
or  in  the  coils  of  the  armature.  To  ascertain  whether  it  be  in  the  latter, 
carefully  examine  the  winding  of  the  faulty  coil. 

The  defect  may  be  sought  for  more  particularly  at  the  commutator 
end  of  the  armature,  as  breaks  in  the  wire  are  most  frequent  where  the 
connections  are  made  with  the  commutator  segments.  If  no  break  can 
be  discovered,  try  passing  a  heax'j'  current  through  the  faulty  coil  by 
means  of  the  brushes. 

If  a  partial  break  exist  with  sufficient  contact  to  pass  a  current,  the 
coil  will  be  heated  at  that  point  and  may  be  discovered  by  running  the 
fingers  over  the  coil. 

When  located,  the  fault  may  be  repaired  by  rewinding  the  coil,  or 
carefully  cleaning  the  broken  ends  and  jointing. 

The  fault  may  also  be  temporarily  repaired  by  soldering  the  adjacent 
commutator  segments  together  without  disconnecting  the  coil. 


CARE  OF  THE   COMMUTATOR  AND  BRUSHES  G35 


CHAPTER  XXXIII 
CARE  OF  THE  COMMUTATOR  AND  BRUSHES 


For  satisfactory  operation,  the  brushes  and  commutator  must 
be  kept  in  good  condition.  To  this  end  the  main  thing  to  l)e 
guarded  against  is  the  production  of  sparks  at  the  brushes.  If 
care  be  taken  in  the  first  instance  to  adjust  the  brushes  to  their 
setting  marks,  and  to  regulate  their  pressure  upon  the  commu- 
tator, and  afterwards  to  attend  to  the  lead  as  the  load  varies,  so 
that  little  or  no  sparking  occurs,  and  also  to  keep  the  brushes 
and  commutator  free  from  dirt,  grit,  excessive  oil,  etc.,  the  sur- 
face of  the  coinmutator  will  assume  a  dark  burnished  appearance 
and  wear  will  practically  cease.  Under  these  circumstances  the 
commutator  will  run  cool,  and  will  give  very  little  trouble. 

In  order  to  maintain  these  conditions  it  will  only  be  neces- 
sary to  see  that  the  brushes  are  kept  in  proper  condition  and 
fed  forward  to  their  setting  marks,  as  they  wear  away,  and 
that  the  commutator  is  occasionally  polished. 

If  the  pressure  of  the  brushes  upon  the  commutator  be  too 
great,  or  their  adjustment  faulty,  or  the  commutator  be  allowed 
to  get  into  a  dirty  condition,  sparking  will  result,  and,  if  not  at 
once  attended  to  and  remedied,  the  brushes  will  quickly  wear 
away,  and  the  surface  of  the  commutator  will  be  destroyed.  As 
this  action  takes  place,  in  the  earlier  stages,  the  surface  of  the 
commutator   will    become   roughened    or   scored,    resulting   in 


636  HAWKINS  ELECTRICITY 

jumping  of  the  brushes,  and  increased  sparking;  in  the  later 
stages,  the  commutator  w-ill  become  untrue  and  worn  into  ruts, 
moreover,  owning  to  the  violent  sparking  which  takes  place 
through  this  circumstance,  the  machine  ■u'ill  quickly  be  rendered 
useless. 

Oues.  How  is  the  commutator  easily  tested  as  to  the 
condition  of  its  surface? 

Ans.  It  is  readily  tested  by  resting  the  back  of  the  finger 
nail  upon  it  while  in  motion;  the  nail  being  very  sensitive  to  any 
irregularities,  indicates  at  once  any  defect. 

Oues.  What  causes  grooves  or  ridges  to  be  cut  in  the 
commutator? 

Ans.  They  result  from  using  brushes  with  hard  burnt  ends 
which  are  not  pliable ;  also  by  too  great  a  pressure  of  the  brush 
upon  the  commutator  surface. 

Sparking  at  the  brushes  is  expensive  and  detrimental,  chiefly  because 
it  results  in  burning  the  brushes  and  also  the  commutator,  necessitating 
their  frequent  renewal.  Ever>'  spark  consumes  a  particle  of  copper, 
torn  from  the  commutator  or  brush.  The  longer  the  sparking  continues, 
the  greater  the  evil  becomes,  and  the  remedy  must  be  applied  without 
delay. 

Oues.  What  kind  of  oil  should  be  used  on  the  com- 
mutator? 

Ans.     Mineral  oil. 

Oues.    What  attention  should  be  given  to  the  brushes? 

Ans.  At  certain  intervals,  according  to  the  care  taken  to 
reduce  sparking  and  the  length  of  time  the  machine  runs,  the 
brushes  will  fray  out  or  wear  unevenly,  and  will  therefore  need 
trimming.  They  should  then  be  removed  from  the  brush  holders 
and  their  contact  ends  or  faces  examined.     If  not  truly  square. 


CARE  OF  THE   COMMUTATOR  AND  BRUSHES 


637 


they  should  be  filed  or  clipped  with  a  pair  of  shears,  the  course 

of  treatment  differing  with  the  type  of  brush. 

If  the  machine  be  fitted  with  metal  strip  brushes,  frayed  ends  should  be 
clipped  square  with  a  pair  of  shears,  the  ends  thoroughly  cleaned  from 
any  dirt  or  carbonized  oil,  and  replaced  in  their  holders.  Gauze  and  wire 
brushes  require  a  little  more  attention.  When  their  position  on  the 
commutator  has  been  well  adjusted  and  looked  after,  so  that  little  or  no 
sparking  has  taken  place,  it  is  generally  only  necessary  to  wipe  them,  clean 
the  brushes  and  clip  off  the  fringed  edges  and  corners  with  the  shears, 
or  a  pair  of  strong  scissors.  If,  however,  the  machine  has  been  sparking, 
the  faces  will  be  worn  or  burnt  away,  and  probably  fused.  If  such  be 
the  case,  they  will  need  to  be  put  in  the  filing  clamp,  and  filed  true. 


Fig.  710. — Bistell  brush  gear.  The  brushes  are  held  in  the  brushholders  radially  and  work 
equally  well  with  armature  running  in  either  direction.  Brushes  can  be  renewed  and 
adjustment  made  while  machine  is  in  operation. 

A  convenient  method  of  trimming  carbon  brushes,  or  of  bedding  a 
complete  new  set  of  metal  brushes,  is  to  bind  a  piece  of  sandpaper,  face 
outwards,  around  the  commutator  after  the  current  has  been  shut  off,  and 
then  mount  the  carbon  or  metal  brushes  in  the  holders,  adjusting  the 
tension  of  the  springs  so  that  the  brushes  bear  with  a  moderately  strong 
pressure  upon  the  sandpaper.  Then  let  the  machine  run  slowlj''  until  the 
ends  of  the  brushes  are  ground  to  the  proper  form.  Care  should  be  taken, 
however,  that  the  metal  dust  given  off  does  not  get  into  the  commutator 
connections  or  armature  windings,  or  short  circuiting  will  result. 

If  the  contact  faces  of  the  brushes  are  very  dirty  and  covered  with  a 
coating  of  carbonized  oil,  etc.,  it  will  be  necessary  to  clean  them  with 
benzoline  or  soda  solution  before  replacing. 


638  HAWKINS  ELECTRICITY 


Oues.     Describe  a  filing  clamp. 

Ans.  As  usually  constructed,  it  consists  of  two  pieces  of 
metal,  both  shaped  at  one  end  to  the  correct  angle,  to  which  the 
brushes  must  be  filed.  One  of  the  pieces  of  metal  (the  back  part) 
has  a  groove  sufficiently  large  to  accommodate  the  brush,  which 
is  clamped  in  position  by  the  other  piece  of  metal  and  a  pinching 
screw. 

If  the  clamp  be  not  supplied  with  the  machine  a  convenient  substitute 
can  be  made  out  of  two  pieces  of  wood  about  the  same  width  as  the 
brush.  One  end  of  each  piece  is  sawn  to  the  correct  angle,  and  the  brush 
placed  between  the  two. 


Fig.  711. — Jig  for  filing  brushes  to  the  correct  bevel;  used  with  copper  brushes  to  fit  them  to 
the  commutator. 


In  filing,  the  brush  is  fixed  in  the  clamp,  with  the  toe  or  tip  projecting 
slightly  over  the  edge  of  the  clamp,  and  the  latter  being  fixed  in  a  vise, 
the  brush  is  filed  by  single  strokes  of  a  smooth  file  made  outwards,  the 
file  being  raised  from  contact  with  the  brush  when  making  the  back 
stroke. 

Sparking. — In  all  well  designed  machines  there  are  certain 
positions  upon  the  commutator  for  the  bnishes  at  which  there  will 
be  no  sparking  so  long  as  the  commutator  is  kept  clean  and  in 
good  condition.  In  other  dynamos,  badly  designed  or  con- 
structed, sparking  occurs  at  all  positions,  no  matter  where  the 


CARE  OF  THE   COMMUTATOR  AND  BRUSHES  639 


brushes  are  placed,  and  in  such  dynamos  it  is  therefore  impossible 
to  prevent  this  no  matter  how  well  they  arc  adjusted. 

Oues.  What  two  kinds  of  sparking  may  be  generally 
distinguished? 

Ans.  One  kind  of  sparking  is  that  due  to  bad  adjustment  of 
the  brushes,  and  a  second  kind,  that  due  to  bad  condition  of  the 
cominutator. 


Fig.  712. — Commutator  clamp;  a  useful  device  for  holding  the  segments  firmly  in  position  in 
taking  out  the  end  rings  of  the  commutator  to  repair  for  internal  grounds.  It  is  made  of 
2  X  J^  inch  sheet  steel,  with  a.  H  inch  screw.  The  illustration  clearly  shows  the  adjustable 
fastening.  The  notches  fit  around  rivets  on  one  side  of  each  fastening,  which  can  be  moved 
by  removing  the  two  cotterg.  The  clamp  is  made  loose  or  taut  by  screwing  the  bolt  in  the 
nut. 

Sparks  due  to  bad  adjustment  of  the  brushes  are  generally  of  a  bluish 
color,  small  when  near  the  neutral  plane,  and  increasing  in  violence  and 
brilliancy  as  the  brushes  recede  from  the  correct  positions  upon  the 
commutator. 

When  sparks  are  produced  by  dirty  or  neglected  state  of  the  com- 
mutator, they  are  distinguished  by  a  reddish  color  and  a  spluttering  or 
hissing.  When  due  to  this  last  mentioned  cause,  it  is  impossible  to  sup- 
press the  sparking  until  the  commutator  and  brushes  have  -been  cleaned. 
In  the  former  case,  the  sparks  will  disappear  as  soon  as  the  brushes  have 
been  rotated  into  the  neutral  points. 

Another  class  of  sparks  appear  when  there  is  some  more  or  less  developed 
fault,  such  as  a  short  circuit,  or  break  in  the  armature  or  commutator. 


640  HAWKINS  ELECTRICITY 

These  are  similar  in  character  to  those  produced  by  bad  adjustment  of  the 
brushes,  but  are  distinguished  from  the  lattter  by  their  not  decreasing 
in  violence  when  the  brushes  are  rotated  towards  the  neutral  plane. 

Ha\-ing  distinguished  the  classes  of  sparks  which  appear  at  the 
commutator  of  a  dynamo,  it  remains  to  enumerate  the  causes 
which  produce  them.    These  are : 

1.  Bad  adjustment  of  brushes; 

2.  Bad  condition  of  brushes; 

3.  Bad  condition  of  commutator; 

4.  Overload  of  dynamo; 

5.  Loose  connections,  terminals,  etc.; 

6.  Breaks  in  armature  circuit; 

7.  Short  circuits  in  armature  circuit; 

8.  Short  circuits  or  breaks  in  field  magnet  circuit. 


Bad  Adjustment  of  Brushes. — When  sparking  is  produced 
by  bad  adjustment  of  the  brushes,  it  may  be  detected  by  rotating 
or  shifting  the  rocker,  by  the  indication  that  the  sparking  vnll 
vary  with  each  movement. 

To  obtain  good  adjustment  of  the  brushes,  it  will  be  necessary 
to  rock  them  gently  backwards  and  forwards,  until  a  position  is 
found  in  which  the  sparking  disappears. 

Oues.  If,  in  rocking  the  brushes,  a  position  cannot  be 
found  at  which  the  sparking  disappears,  what  is  the  prob- 
able cause  of  the  trouble? 

Ans.  The  brushes  may  not  be  set  with  the  proper  pitch,  that 
is  they  may  not  be  separated  a  correct  distance,  or  the  neutral 
plane  may  not  be  situated  in  the  true  theoretical  position  upon 
the  commutator  through  some  defect  in  the  winding,  etc. 

In  this  last  named  case,  the  brushes  may  be  strictly  adjusted  to  their 
theoretically  correct  positions  before  starting  the  machine ;  then,  wheo 


C4RE  OF  THE  COMMUTATOR  AND  BRUSHES 


641 


the  machine  is  started  and  the  load  put  on,  violent  sparking  occurs,  which 
cannot  be  suppressed  by  shifting  the  rocker.  If,  however,  one  set  of 
brushes  only  be  observed,  it  will  generally  be  found  that,  at  a  certain 
position,  the  sparking  at  the  set  of  brushes  under  observation  ceases  or 
is  greatly  reduced,  while  sparking  still  occurs  at  the  other  set.  When 
this  position  is  found,  the  rocker  should  be  fixed  by  the  clamping  screw, 
and  the  brushes  of  the  other  set  at  which  sparking  is  still  occurring 
adjusted  by  drawing  them  back  or  pushing  them  forward  in  their  holders 
until  a  position  is  found  at  which  the  sparking  ceases.  Correct  position 
of  the  brushes  and  the  suppression  of  sparking  is  a  matter  of  importance, 
and  any  time  spent  in  carefully  adjusting  the  brushes  will  be  amply 
repaid  by  the  decreased  attention  and  wear  of  the  brushes  and  com- 
mutator. 


Figs.  713  to  715. — Brushes  making  bad  contact.  A  brush  making  a  bad  contact,  as  only  at 
the  shaded  portion  of  figs.  713  and  714,  will  not  allow  the  short  circuited  coil  enough  time 
to  reverse,  causing  sparking  and  heating.  The  latter  will  also  result  from  bad  contact  on 
account  of  the  surface  being  too  small  for  the  current  to  be  carried  off.  This  form  of  bad 
contact  is  worse  than  that  shown  in  fig.  715,  where  the  area  of  contact  surface  only  is 
lessened.    If  the  brushes  do  not  make  good  contact,  they  should  be  ground  down. 

Bad  Condition  of  Brushes.- — If  the  contact  faces  of  the 
brushes  be  fused  or  covered  with  carbonized  oil,  dirt,  etc.,  there 
will  be  bad  contact  which  is  accompanied  by  heating  and  spark- 
ing. Simple  examination  will  generally  reveal  whether  this  be 
the  case.  The  remedy  is  to  remove  the  brushes,  one  at  a  time 
if  the  machine  be  running,  clean,  file  if  necessary,  trim,  and 
readjust. 

If  the  brushes  be  exceedingly  dirty,  or  saturated  with  oil,  it 
will  be  necessary  to  clean  them  with  turpentine,  benzoline,  or 
soda  solution,  before  replacing. 


Bad  Condition  of  Commutator. — If  the  surface  of  the 
commutator  be  rough,  worn  into  grooves,  or  eccentric,  or  if 
there  be  one  or  more  segments  loose  or  set  irregularly,  the 


642 


HAWKINS  ELECTRICITY 


brushes  "vsill  be  throvi-n  into  ^-ib^ation,  and  sparking  vriR  result. 
A  simple  examination  of  the  commutator  will  readily  detect 
these  defects.  A  rough  and  uneven  commutator  is  due  to  bad 
adjustment  of  brushes,  bad  construction  of  commutator,  and  to 
neglect  generalh',  If  allowed  to  continue,  it  results  in  hea\'>' 
sparking  at  the  brushes,  and  the  eventful  destruction  of  the 
commutator.  The  fault  may  be  remedied  by  filing  or  re-ttuTiing 
the  commutator. 


Pig.  716. — Rou^  and  grooved  oommutator  due  to  impnaper  brush  cidjustment  and  failure  to 
keep  brashes  in  pitiper  ooDditaoa. 


Ques.     How  is  an  untrue  commutator  detected? 

Ans.  If  the  commutator  be  untrue,  the  fact  will  be  indicated 
when  the  machine  is  slowed  down  by  a  \-isible  eccentricity,  or 
b}'  holding  the  hand,  or  a  stick  in  the  case  of  a  high  tension 
machine,  against  the  surface  while  revohdng,  when  any  irregu- 
larity or  eccentricity  will  be  apparent  by  the  \-ibration  or 
movement  of  the  stick.  The  only  remedy  for  an  imtrue  com- 
mutator is  to  re-tum  it  in  the  lathe. 


CARE  OF  THE   COMMUTATOR  AND  BRUSHES  643 


Oues.    What  should  be  done  in  case  of  high  segments? 

Ans.  They  should  be  gently  tapped  down  with  a  mallet,  and 
if  possible  the  clamping  cones  at  the  commutator  end  should 
be  tightened. 

If  it  be  impossible  to  hammer  the  segments  down,  they  should  be  filed 
down  to  the  same  diameter  as  the  rest  of  the  commutator,  or  the  com- 
mutator re-turned.  For  low  segments,  the  only  remedy  is  to  pull 
out  the  segments,  or  turn  commutator  down  to  their  level. 

Oues.     Explain  the  term  "  flats  on  the  commutator." 

Ans.  This  is  the  name  given  to  a  peculiar  fault  which  develops 
on  one  or  more  segments  of  the  commutator.  It  is  not  confined 
to  dynamos  of  bad  design  or  construction,  but  frequently  appears 
on  those  of  the  highest  class,  and  may  be  recognized  as  a  "  pit- 
ting "  or  "  flattening  "  of  one  or  more  segments. 

Oues.    What  is  the  effect  of  flats  on  the  commutator? 

Ans.     Sparking  at  the  brushes. 

Oues.    What  are  the  causes  which  produce  flats? 

Ans.  Periodical  jumping  of  the  brushes  due  to  a  bad  state 
of  the  commutator,  bad  joint  in  the  driving  belt,  a  flaw,  or  a 
difference  in  the  composition  of  the  metal  of  the  particular  bar 
upon  which  it  appears.  But  more  frequently  flats  may  be 
traced  to  a  more  or  less  developed  fault,  such  as  a  break,  either 
partial  or  complete,  in  the  armature  coil. 

The  break  may  occur  either  in  the  coil  itself,  or  at  the  point  where  its 
ends  make  connection  with  the  lug  of  the  commutator,  or  at  the  point 
where  the  lug  is  soldered  to  the  segment. 

Oues.    What  should  be  done  in  case  of  flats? 

Ans.  The  brushes  should  be  examined  to  see  if  any  periodical 
vibration  take  place.  If  such  be  the  case,  the  cause  should  be 
removed,  the  flat  carefully  filed  or  turned  out,  and  the  brushes 
readjusted. 


644 


HAWKINS  ELECTRICITY 


If  it  be  due  to  a  difference  in  the  composition  of  the  metal  of  which 
the  segment  is  made,  the  fiat  will  exist  as  long  as  the  particular  segment 
is  in  use,  and  will  need  periodic  attention. 

With  hard  drawn  copper  or  phosphor  bronze  segments,  this  fault  is 
rarely  due  to  this  last  mentioned  cause.  It  is  more  frequently  due  to 
bad  soldering,  of  the  conductors  to  the  lugs,  or  of  the  lugs  to  the  segments. 
In  all  cases  of  flats,  if  the  disconnection  in  the  armature  circuit  be  not 
complete,  and  cannot  be  readily  located,  the  effect  of  re-soldering  or 
sweating  the  ends  of  the  coils  into  the  lugs  should  be  tried.  Flats  may 
also  frequently  be  cured  by  drilling  and  tapping  a  small  hole  in  the 
junction  between  the  lug  and  the  segment,  and  inserting  a  small  screw, 
or  bit  of  screwed  copper  or  brass  wire,  afterwards  filing  down  level  with  the 
surface  of  the  commutator. 


I  n  J  f,n  ^ 


Figs.  717  and  71S. — Method  of  repairing  broken  joint  between  commutator  segment  and  lug. 
To  repair  such  a  break  push  asbestos  in  between  adjacent  bars,  so  that  heat  from  the  torch 
will  not  affect  them.  Asbestos  should  also  be  worked  in  at  the  back  if  possible,  for  the 
purpose  of  keeping  solder  from  places  where  it  would  cause  trouble.  Then  unsolder  the 
armature  leads  from  the  lug  and  remove  the  latter.  Next,  with  specially  made  cape  chisels, 
cut  in  a  slot  in  the  commutator  bar  for  a  new  lug.  Care  and  skill  are  required  not  to  de- 
stroy the  mica  insulation  between  the  segments.  The  slot  should  be  cut  one-quarter  to 
three-eighths  inch  deep.  The  connector  is  then  soldered  in  place.  With  care  a  satisfac- 
tory-connection can  be  made  in  this  way,  which  will  last  well.  If  it  do  not  last,  the  trouble 
in  almost  every  case  is  due  to  poor  soldering.  Short  circuits  sometimes  occur  after  this 
operation,  because  of  solder  falling  in  at  the  back  and  lodging  on  lower  connections.  In 
large  machines,  the  excessive  current  flowing  is  quite  likeljtto  melt  this  solder,  and  the 
machine  may  buck,  throwing  out  the  melted  solder,  after  which  it  may  be  all  right  again. 
While  the  bar  connector  is  out,  however,  asbestos  should  be  packed  in  back  of  it  to  pre- 
vent this  occurrence,  which  may  be  a  serious  affair.  All  surplus  solder  and  the  asbestos 
packing  should  be  removed  after  the  connection  is  finished,  and  the  connections  cleaned 
with  compressed  air.  The  armature  should  be  turned  over  slowly,  air  being  applied  all 
the  while. 

Segments  Loose  or  Knocked  In. — When  the  segments  are 
loose,  it  is  an  indication  that  the  clamping  ring  or  cone  has  worked 
loose.  This  should  therefore  be  tightened  up,  and  the  commutator 
r©-tumed  if  necessary. 


CARE  OF  THE   COMMUTATOR  AND  BRUSHES  645 

Ques.  How  should  low  commutator  segments  be 
treated  ? 

Ans.  The  commutator  surface  may  be  turned  down  to  the 
level  of  the  low  segment,  or  the  latter  may  be  pulled  out  again 
to  its  former  level,  this  latter  being  the  preferable  method,  if  it 
can  possibly  be  effected. 

Oues.  How  is  a  commutator  segment  pulled  out  to  its 
correct  position? 

Ans.  A  hand  vise  is  firmly  clamped  to  the  lug,  or  a  loop  of 
copper  wire  is  passed  round  the  conductor  where  it  joins  the 
commutator.  A  bar  of  iron,  to  act  as  a  lever,  is  supported  on  a 
fulcrum  over  the  commutator,  and  one  end  of  the  bar  is  passed 
through  the  loop  or  vise.  Pressure  is  applied  to  the  other  end 
which  will  generally  bury  the  segment  up  to  its  proper  position. 


How  to  Re-turn  a  Commutator. — In  re-turning  the  com- 
mutator, the  armature  should  first  be  carefully  taken  out  of  the 
armature  chamber,  avoiding  knocks  or  blows  of  any  kind.  The 
whole  of  the  winding  should  then  be  wrapped  in  calico  or 
canvas  before  the  armature  is  put  into  the  lathe,  to  prevent 
any  particles  of  metal  becoming  attached  to  the  surface  of  the 
armature  at  the  time  the  commutator  is  being  turned.  The 
armature  should  on  no  account  be  rolled  upon  the  floor,  or 
subjected  to  blows  or  knocks  while  being  put  into  the  lathe. 

In  re-turning  the  commutator,  a  sharp  pointed  tool  should  be  used 
with  a  very  fine  feed.  A  broad  nosed  tool  ought  not  to  be  used,  as  it  is 
liable  to  burr  over  the  segments.  After  turning,  the  commutator  should 
be  lightly  filed  with  a  dead  smooth  file,  and  finally  polished  with  coarse 
and  fine  sandpaper.  After  the  commutator  has  been  turned  and  polished, 
the  insulation  between  the  segments  should  be  lightly  scraped  with  the 
tang  of  a  small  file  to  remove  any  particles  of  metal  or  burrs  which  might 
short  circuit  the  commutator. 


646 


HAWKINS  ELECTRICITY 


The  points  where  the  armature  ■n"ires  are  soldered  to  the  lugs  should 
also  be  carefully  cleaned  with  a  brush,  and  should  then  receive  a  coat 
or  two  of  shellac  varnish. 

While  the  commutator  is  being  turned,  care  should  be  taken  that  the 
setting  marks  for  the  adjustment  of  the  brushes  are  not  turned  out  if 
these  be  present.  The  same  care  should  be  used  in  putting  the  armature 
back  into  the  armature  chamber  as  was  used  in  taking  it  out,  otherwise 
the  insulation  may  be  damaged. 


Figs.  719  and  720. — Bissell  commutators.    The  segments  are  of  hard  drawn  copper  and  are 
insulated  from  each  other  and  from  the  shell  by  mica. 


Ques.  Should  the  commutator  be  run  without  any 
lubricant? 

Ans.  In  most  cases  it  will  be  found  that  a  little  lubricant 
is  needed  in  order  to  prevent  cutting  the  brushes,  cutting  the 
commutator ;  this  is  especially  the  case  when  hard  strip  brushes 
are  used.  The  quantity  of  oil  applied  should  be  ver}^  small;  a 
few  drops  smeared  upon  a  piece  of  clean  rag,  and  applied  to  the 
commutator  while  running,  being  quite  sufficient. 

Ques.  What  kind  of  oil  should  be  used  on  the  com- 
mutator? 

Ans.  Mineral  oil,  such  as  vaseline,  or  any  other  hydro- 
carbon.    Animal  or  vegetable  oils  should  be  avoided,  as  they 


CARE  OF  THE   COMMUTATOR  AND  BRUSHES 


047 


have  a  tcndenc}''  to  carbonize,  and  thus  cause  short  circuiting 
of  the  commutator,  with  attendant  sparking. 

Overload  of  Dynamo. — It  may  happen,  through  some  cause 
or  other  that  a  greater  output  is  taken  from  the  machine  than 
it  can  safely  carry.  When  this  is  the  case,  the  fact  is  indicated 
by  excessive  sparking  at  the  brushes,  great  heating  of  the  armature 


Figs.  721  to  72.3. — Method  of  repairing  a  large  hole  burned  in  two  adjacent  bars  of  a  com- 
mutator. Fig.  721  shows  the  hole.  The  first  operation  is  to  clean  carefully  and  tin  the  sur- 
face of  the  hole.  Thetwobarsare  then  wedged  apart  and  mica  strips,  A  B,  fig  722,  of  the  re- 
quisite size  and  thickness  forced  in.  The  commutator  must  now  be  warmed  up  as  much  as 
possible  by  means  of  soldering  irons,  and  strips  of  mica,  C  D,  E  F,  fig.  723,  placed  at  the 
front  and  back  of  the  hole,  being  kept  in  position  by  pieces  of  wood  W,  solder  is  poured 
into  the  hole  from  a  ladle,  using  a  rough  mica  funnel  to  guide  it. 

and  other  parts  of  the  dynamo,  and  possibly  by  the  slipping 
of  the  belt  (if  it  be  a  belt  driven  machine),  resulting  in  a  noise. 
The  causes  most  likely  to  produce  overload  are: 

NOTE. — In  operating  dynamos  having  metal  brushes,  it  is  of  importance  to  keep  the 
commutator  smooth  and  glossy.  To  accomplish  this,  it  is  necessary  to  keep  the  commutator 
and  brushes  clean  and  free  from  grit,  and  to  occasionally  lubricate  the  commutator  with  some 
light  oil,  such  as  ordinary  machine  oil.  1  his  should  be  done  da'ly  if  the  machine  be  in  constant 
use.  Keep  the  brushes  resting  upon  the  commutator  with  just  enough  pressure  to  insure  a 
good  firm  contact.  This  will  be  found  to  be  much  less  than  the  springs  are  capable  of  exerting. 
A  good  method  to  follow  i  n  cleaning  the  machine  is  as  follows:  Loosen  the  brush  holder  thumb 
screws  and  tilt  the  brushes  off  the  commutator  (or,  if  box  brush  holders  be  used,  take  them 
out  of  their  holders).  Then  run  the  machine  and  hold  a  clean  cloth  against  the  commutator. 
After  the  commutator  is  clean,  hold  against  it  a  cloth  or  piece  of  waste  moistened  with  machine 
oil  and  reset  the  brushes.  If  for  any  reason  the  brushes  begin  to  cut  or  score  the  commutator, 
it  may  be  readily  detected  by  holding  the  finger  against  the  commutator;  the  ridge  may  be 
easily  felt  by  the  finger.  This  should  be  attended  to  at  once  in  the  following  manner:  Tilt  back 
the  brushes  (or  if  box  brushes  are  used  take  them  out  of  their  holders),  and  hold  lightly  against 
the  commutator  a  piece  of  No.  00  sandpaper  well  moistened  with  oil,  passing  it  back  and  forth 
until  the  surface  is  perfectly  smooth.  Then  wipe  off  the  commutator  with  a  clean  piece  of 
cloth  or  waste  and  lubricate  with  another  clean  piece  moistened  with  oil  and  reset  the  brushes. 


648 


HAWKINS  ELECTRICITY 


1 .  Excessive  voltage ; 

2.  Excessive  current ; 

3.  Reversal  of  polarity  of  dynamo; 

4.  Short  circtiits  or  groiinds  in  dynamo,  or  external  circuits. 

Oues.     What  is  the  indication  of  excessive  voltage? 

Ans.     It  is  indicated  by  the  voltmeter,  or  by  the  brilliancy  of 
the  pilot  lamp. 


Fig.  724. — Method  of  smoothing  commutator  with  a  stone.  The  proper  stone  to  use  is  made 
out  of  white  sandstone  similar  to  that  used  for  grindstones,  but  a  trifle  softer.  It  is  dove- 
tailed into  a  holder,  as  shown  in  the  illustration,  and  h«ld  in  place  by  a  set  screw.  'When 
being  used,  one  knob  is  grasped  in  one  hand  and  the  other  knob  in  the  other  hand,  the  stone 
being  moved  back  and  forth  along  the  length  of  the  commutator.  As  the  stone  will 
become  coated  with  copper  at  first,  it  must  be  cleaned  freque.jtly  ty  means  of  coarse 
sandpaper.  The  fine  dust  from  the  stone  will  get  under  the  brushes  and  wear  them  to  a 
very  close  fit.    After  using  the  stone,  finish  with  fine  sandpaper. 


Oues.    What  are  the  causes  of  excessive  voltage? 

Ans.     Over  excitation  of  the  field  magnet  or  too  high  speed. 

In  the  former  case,  resistance  should  be  introduced  into  the  field  cir- 
cuit to  diminish  the  current  flowing  therein  if  a  shunt  machine:  or  if  a 
series  machine,  a  portion  of  the  current  should  be  shunted  across  the 
field  coils  by  means  of  a  resistance  arranged  in  parallel  with  the  series 


CARE  OF  THE   COMMUTATOR  AND  BRUSHES  649 

coils;  or  the  same  effect  may  be  produced  in  both  cases  by  reducing  the 
speed  of  the  armature  if  this  be  possible. 

If  due  to  excessive  speed,  which  will  be  indicated  by  a  speed  indicator, 
the  natural  remedy  is  to  reduce  the  speed  of  the  engine  driving  the 
dynamo,  or,  if  this  be  not  easily  done,  insert  resistance  into  the  dynamo 
circuit,  as  described  above. 

Ques.    What  are  the  causes  of  excessive  current? 

Ans.  If  the  dynamo  be  supplying  arc  lamps,  the  excessive 
current  may  possibly  be  caused  by  the  bad  feeding  of  the  lamps. 
If  this  be  the  case,  the  fact  will  be  indicated  by  the  oscillations 
of  the  ammeter  needle,  and  the  unsteadiness  of  the  light. 

If  incandescent  lamps  be  in  the  circuit,  the  fault  may  be  caused  by 
there  being  more  lamps  in  circuit  than  the  dynamo  is  designed  to  carry. 
Under  such  circumstances,  another  djmamo  should  be  switched  into 
circuit  in  parallel,  or,  if  this  be  not  possible,  lamps  should  be  switched  off 
until  the  defect  is  remedied. 

When  motors  are  in  the  circuit,  sparking  frequently  results  at  the 
dynamo  commutator,  owing  to  the  fluctuating  load.  In  such  cases  the 
brushes  should  be  adjusted  to  a  position  at  which  the  least  sparking 
occurs  with  the  average  load. 

Ques.  What  may  be  said  with  respect  to  reversal  of 
polarity  of  dynamos? 

Ans.  When  compound  or  scries  wound  dynamos  are  running 
in  parallel,  their  polarity  is  occasionally  reversed  while  stopping 
by  the  current  from  the  machines  at  work. 


Loose  Connections,  Terminals,  etc. — When  any  of  the 
connecting  cables,  terminal  screws,  eic.,  securing  the  different 
circuits  are  loose,  sparking  at  the  brushes,  as  a  rule,  results,  for 
the  reason  that  the  vibration  of  the  machine  tends  to  continually 
alter  the  resistance  of  the  various  circuits  to  which  they  are 
connected. 

When  the  connections  are  excessively  loose,  sparking  also 
results  at  their  points  of  contact,  and  by  this  indication  the 


650 


HAWKINS  ELECTRICITY 


faulty  connections  may  be  readily  detected.  When  this  sparking 
at  the  contacts  is  absent,  the  whole  of  the  connections  should  be 
carefully  examined  and  tested. 

Breaks  in  .\rinature  Circuit. — If  there  be  a  broken  circuit 
in  the  armature,  as  sometimes  happens  through  a  fracture  of 
the  armature  connections,  etc.,  there  will  be  serious  flashing  or 
sparking  at  the  brushes,  which  cannot  be  suppressed  by  adjust- 
ing the  rocker.  As  a  rule  it  results  in  the  production  of  "  flats  " 
upon  one  or  more  bars  of  the  commutator. 


Fig.  725. — Sandpaper  holder  for  commutator.  The  sandpaper  is  made  fast  on  top  by  a  clamp 
and  screw.  The  two  face  blocks  are  pivoted  and  adjust  themselves  to  the  commutator, 
and  will  fit  any  size  of  commutator.  If  it  have  four  brushes,  the  lower  block  vnU  go  in  be- 
tween the  brush-holders. 

Oues.     How  may  such  sparking  be  reduced  without 
stopping  the  machine? 

Ans.     By  placing  one  of  the  brushes  of  each  set  a  little  in 
advance  of  the  others,  so  as  to  bridge  the  gap. 


Short  Circuits  in  Armature  Circuit. — This  fault  is  indi- 
cated by  sparking  at  the  commutator,  and  in  bad  cases  by  an 
excessive  heating  of  the  armature,  dimming  of  the  light  and 


CARE  OF  THE   COMMUTATOR  AND  BRUSHES  651 

slipping  of  the  belt,  and  in  the  case  of  a  drum  armature,  by  a 
sudden  cessation  of  the  current. 


Short  Circuits  or  Breaks  in  Field  Magnet  Circuit. — 

Either  of  these  faults  is  liable  to  give  rise  to  sparking  at  the 
commutator.  If  one  of  the  coils  be  short  circuited,  the  fact  will 
be  indicated  by  the  faulty  coil  remaining  cool  while  the  perfect 
coil  is  overheated.     The  fault  may  arise  through  some  of  the 


Fig.  726. — Saxidpaper  block.  It  is  made  to  fit  the  surface  of  the  commutator.  At  S  is  a  saw 
cut  into  which  the  ends  are  pushed  after  being  wrapped  around  the  block.  The  latter 
should  be  cut  down  on  the  dotted  lines  to  form  a  handle. 


connections  to  the  coils  making  contact  with  the  frame  of  the 
machine  or  with  each  other.  To  ascertain  this,  examine  all  the 
connections,  and  test  with  a  battery  and  galvanometer.  A  total 
break  in  one  or  more  of  the  field  coils  may  readily  be  detected  by 
means  of  the  battery  and  galvanometer. 

A  partial  break  is  not,  however,  so  readily  discovered,  for  the  reason 
that  the  coil  wires  may  be  in  sufficiently  close  contact  to  give  a  deflection 
of  the  galvanometer  needle.  The  only  methods  of  detecting  this  fault 
is  by  measuring  the  resistance   of    the  coils  with  an  ohmmeter  or 


652  HAWKINS  ELECTRICITY 


"VMieatstone  bridge,  or  by  placing  an  ammeter  in  circuit  with  each  coil 
in  turn,  and  comparing  the  amount  of  current  flowing  in  each.  If  the 
partial  break  be  not  accessible,  the  only  way  to  remedy  the  fault  is  to 
rewind  the  coil,  and  the  same  applies  to  a  break  in  the  interior  of 
the  coil. 

Short  Circuits  in  Cominutator. — These  are  of  frequent 
occurrence,  and  result  in  heating  the  armature  and  sparking 
at  the  brushes.  They  are  caused  either  by  metallic  dust  ot 
particles  lodging  in  the  insulation  between  the  segments,  or  b> 
the  deterioration  of  the  commutator  insulation. 

To  remedy,  the  insulation  between  the  segments  should  be  carefully 
examined,  and  any  metallic  dust,  filings,  or  burrs  cleaned  or  scraped  out. 
When  the  commutator  is  insulated  with  asbestos  or  pasteboard  (as  is 
oftentimes  the  case  in  dj'namos  of  European  make),  short  circuits  ver}- 
frequently  occur  through  the  insulation  absorbing  moisture  or  oil,  which 
is  subsequently  carbonized  by  the  sparking  at  the  brushes.  In  faults  of 
this  description  the  only  remedy  is  to  expel  all  moisture  from  the  com- 
mutator insulation  by  means  of  heat,  and  scrape  out  all  metallic  dust 
which  may  be  embedded  in  the  surface  of  the  insulation.  If  this  do  not 
effect  a  cure,  it  will  be  necessary  to  dig  out  the  insulation,  as  far  as  pos- 
sible, with  a  sharp  tool,  and  drive  in  new  insulation.  Oil  should  not  be 
used  on  commutators  insulated  ^\ith  these  materials,  but  only  asbestos 
dust  or  French  chalk. 


HEA  TING  653 


CHAPTER  XXXIV 
HEATING 


The  excessive  heating  of  the  parts  of  dynamos  and  motors  is 
probably  the  most  frequent  and  annoying  fault  which  arises  in 
operation.  When  the  machine  heats,  it  is  a  common  mistake  to 
suppose  that  any  part  fotmd  to  be  hot  is  the  seat  of  the  trouble. 
Hot  bearings  may  cause  the  armature  or  commutator  to  heat, 
or  vice  versa. 

All  parts  of  the  machine  should  be  tested  to  ascertain  which 
is  the  hottest,  since  heat  generated  in  one  part  is  rapidly  diffused. 
This  is  best  done  by  starting  with  the  machine  cold;  any  serious 
trouble  from  heating  is  usually  perceptible  after  a  run  of  a  few 
minutes  at  full  speed  with  the  field  magnets  excited. 

Heating  may  be  due  to  various  electrical  or  mechanical  causes, 
and  it  may  occur  in  the  different  parts  of  the  machine,  as  in: 

1.  The  connections; 

2.  The  brushes  and  commutator; 

3.  The  armature; 

4.  The  field  magnet; 

5.  The  bearing. 

Oues.     How  is  heating  detected  ? 

Ans.  By  api)lying  the  hand  to  the  different  parts  of  the 
machine  if  low  tension,  or  a  thermometer  if  high  tension,  and 
also  by  a  smell  of  overheated  insulation,  paint,  or  varnish. 


654  HAWKINS  ELECTRICITY 


Oues.  What  should  be  done  if  the  odor  of  overheated 
insulation,  paint  or  varnish  be  noticeable? 

Ans.  It  is  advisable  to  stop  the  machine  at  once,  otherwise 
the  insulation  is  liable  to  be  destroyed. 

Oues.  What  is  the  allowable  rise  of  temperature  in  a 
well  designed  machine? 

Ans.  It  should  not  exceed  80°  Fahr.,  above  the  surrounding 
air,  and  in  the  case  of  the  bearings,  this  temperature  ought  not 
to  be  reached  under  normal  conditions  of  working. 

If  this  limit  be  exceeded  after  a  run  of  six  hours  or  less,  it  indicates  a 
machine  either  badly  designed  and  probably  with  the  material  cut  down 
to  the  lowest  possible  limit  with  a  view  to  cheapness,  or  some  fault  or 
other  which  should  be  searched  for  and  remedied  as  early  as  possible, 
otherwise  the  machine  will  probably  be  destroyed. 

Oues.     How  should  the  rise  of  temperature  be  measured  ? 

Ans.  It  is  not  sufficient  to  feel  the  machine  with  the  hand, 
but  special  thermometers  must  be  placed  on  the  armature  wind- 
ing, immediately  on  stopping  the  machine,  covering  them  with 
cotton  or  wool  to  prevent  cooling.  Readings  must  be  taken  at 
short  intervals,  and  continued  till  no  further  rise  of  temperature 
is  indicated. 

Heating  of  Connections. — A  rise  of  temperature  of  the 
connections  may  be  due  to  either  excessive  current,  or  bad  con- 
tacts, or  both.  The  terminals  and  connections  will  be  excessively 
heated  if  a  larger  current  pass  through  them  than  they  are  de- 
signed to  carry.  This  nearly  always  proceeds  from  an  overload 
of  the  dynamo,  and  if  this  be  rectified,  the  heating  will  disappear. 

If  the  contacts  of  the  diflfcrcnt  connections  of  the  dynamo  be  not  kept 
thoroughly  clean  and  free  from  all  grit,  oil,  etc.,  and  the  connections 
themselves  be  not  tightly  screwed  up,  heating  will  result,  and  the  con- 
nections may  even  become  unsoldered. 


HEATING 


656 


Heating   of   Brushes,    Commutator   and   Armature. — 

When  heating  occurs  in  these  parts,  it  may  be  due  to  any  of  the 
following  causes:  1,  excessive  current;  2,  hot  bearings;  3,  short 
circuits  in  armature  or  commutator;  4,  moisture  in  armature 
coils;  5,  breaks  in  armature  coils;  6,  eddy  currents  in  armature 
core  or  conductor. 


Fig.  727. — Ventilated  commutator;  sectional  view  showing  air  ducts.  Air  is  frequently  circu- 
lated through  a  commutator  in  order  to  maintain  it  at  a  sufficiently  low  temperature, 
suitable  openings  being  provided  for  this  purpose  as  shown. 


Oues.  What  may  be  said  with  respect  to  excessive 
current? 

Ans.  "When  a  dynamo  is  overloaded,  the  temperature  of  the 
armature  will  rise  to  a  dangerous  extent,  depending  upon  the 
degree  to  which  the  safe  capacity  of  the  machine  is  exceeded,  and 
heavy  sparking  of  the  brushes  will  also  result.  If  the  overload 
be  not  removed,  the  insulation  of  the  armature  may  be  destroyed. 


656 


HAWKINS  ELECTRICITY 


Ques.     State  some  causes  of  hot  bearings. 

Ans.  Lack  of  oil ;  presence  of  grit  or  other  foreign  matter  in 
the  bearings;  belt  too  tight;  armature  not  centred  with  respect 
to  pole  pieces;  bearings  too  tight;  bearings  not  in  line;  shaft 
rough  or  cut. 


Fig.  72S. — Self-oiling  and  self-aligning  bearing.  The  self-oiling  feature  consists  of  rings  which 
revolve  with  the  shaft,  and  feed  the  latter  with  oil  continually,  which  they  bring  up  from 
the  reservoir  below.  The  dirt  settles  to  the  bottom,  and  the  upper  portion  of  the  oil 
remains  sufficiently  clean  for  a  long  time,  after  which  it  is  drawn  off,  and  a  fresh  supply 
poured  in  through  holes  pro^^ded  in  the  top.  These  latter  are  often  located  directly 
over  the  slots  in  which  the  rings  are  placed,  so  that  the  bearings  can  be  lubricated  imme- 
diately by  means  of  an  oil  cup  if  the  rings  fail  to  act  or  the  reser\-oir  become  exhausted. 
The  bearing  is  made  self-aligning  by  pro\-iding  the  bearing  proper  with  an  enlarged  cen- 
tral portion  of  spherical  shape,  held  in  a  spherical  seat  formed  in  the  pedestal  by  turning, 
milling,  or  by  casting  Babbitt  or  other  fusible  metal  around  it,  thus  allowing  the  bearing 
to  adjust  itself  to  the  exact  direction  of  the  shaft.  The  upper  half  of  the  box  can  be 
taken  off  to  facilitate  renewal,  etc.,  and  to  permit  the  armature  to  be  removed. 


Oues.     What  is  the  eifect  of  hot  bearings? 

Ans.  Besides  giving  trouble  themselves,  the  heat  may  be 
conducted  along  the  armature  shaft  and  core,  thus  gi\'ing  rise 
to  excessive  heating  of  the  armature. 


HEATING  657 


POINTS  RELATING  TO  HOT  BEARINGS 

1 .  Use  good  oil ; 

2.  See  that  oil  cups  or  reservoirs  are  full  and  all  oil  passages  clear; 

3.  In  self-oiling  and  splash  systems  where  the  oil  is  used  over  again,  it 
should  be  kept  in  clean  condition  by  frequent  straining; 

4.  Keep  bearings  clean  and  properly  adjusted; 

5.  Maintain  bearings  in  good  alignment; 

6.  Avoid  tight  belts; 

7.  Examine  the  air  gap  or  clearance  between  armature  and  pole 
faces  and  see  that  they  are  uniform. 

Oues.  What  troubles  are  encountered  with  short 
circuits  in  the  armature  or  commutator? 

Ans.  This  results  in  sparking  at  the  brushes,  and  in  the 
heating  of  one  or  more  of  the  armature  coils,  and  even  in  the 
burning  up  of  the  latter  if  a  bad  case. 

"When  the  armature  is  overheated,  and  the  defect  does  not  proceed 
from  an  overload  or  the  causes  mentioned  below,  the  dynamo  should  be 
immediately  stopped  and  tested  for  this  fault. 

Oues.  What  will  happen  with  an  overheated  com- 
mutator? 

Ans.  It  will  decompose  carbon  brushes  and  cover  the  com- 
mutator with  a  black  film,  which  offers  resistance  and  increases 
the  heat. 

Oues.  What  should  be  done  if  carbon  brushes  become 
hotter  than  the  other  parts? 

Ans.  Use  higher  conductivity  carbon.  Reduce  length  of 
brush  by  adjusting  holder  to  grip  brush  nearer  the  commutator. 
Reinforce  brushes  with  copper  gauze,  sheet  copper  or  wires,  or 
use  some  form  of  combined  metal  and  carbon  brush.  Increase 
size  or  number  of  brush  if  necessary,  so  the  current  does  not 
exceed  30  amperes  per  square  inch  of  contact. 


658 


HAWKINS  ELECTRICITY 


Brushes  heat  sometimes  due  to  too  much  friction.    They  should  not 
press  against  the  commutator  more  than  is  necessan.'  for  good  contact. 

Ques.     Give  some  causes  for  heating  of  armature. 

Ans.  Eddy  currents;  moisture;  short  circuits;  unequal 
strength  of  magnetic  poles ;  operation  above  rated  voltage,  and 
below  normal  speed. 

Ques.     What  trouble  is  encountered  with  eddy  currents  ? 

Ans.  Considerable  heating  of  the  whole  of  the  armature 
results,  which  may  even  extend  to  the  bearings. 


Fig.  729. — Eck  Manchester  type  motor.  It  is  a  ver\'  small  size  unit  and  is  designed  for  special 
purposes  where  very  little  room  is  available.  The  motor  occupies  a  space  of  2  J^"  X  4  J^" 
between  bearings  and  develops  A  horse  power  at  2.000  R.  P.  M.  The  frame  of  this  motor 
is  miade  of  high  permeability  steel  so  as  to  reduce  the  weight  to  a  minimum.  The  armature 
is  of  the  hand  wound  bipolar  t\-pe  built  up  of  thin  punchings.  The  armature,  after  being 
wound,  is  baked  at  high  temperature  for  a  prolonged  period  and  then  dipped  while  hot  in 
insulating  varnish.  Pully  is  one  inch  in  diameter  and  takes  a  }-4  inch  round  belt.  Weight 
of  motor  5  J-2  iMunds. 


Ques.     How  can  this  be  overcome? 

Ans.     There  is  no  remedy  for  eddy  currents  other  than  the 
piirchase  of  a  new  armature,  or  reconstruction. 

The  fault  may  be  detected  bj^  exciting  the  field  magnets  and  running 
the  machine  on  open  circuit,  with  the  brushes  raised  off  the  commutator 
for  some  time,  when  the  armature  will  be  fovmd  to  be  excessively  heated. 


HEATING  659 


Ques.  How  does  moisture  in  the  armature  coils  affect 
the  armature? 

Ans.  The  effect  of  this  fault  being  to  practically  short  circuit 
the  armature,  a  heating  of  the  latter  results.  In  bad  cases,  steam 
or  vapor  is  given  off. 

Ques.  What  is  the  effect  of  short  circuits  in  the  arma- 
ture? 

Ans.     It  produces  overheating. 

Oues.  What  trouble  is  likely  to  occur  when  the  arma- 
ture is  not  centered  in  the  armature  chamber? 

Ans.  A  heating  of  the  bearings  is  liable  to  be  occasioned 
through  the  attractive  forces  developed  by  the  center  of  the 
armature  core  not  being  parallel  with  the  centre  of  the  armature 
chamber  or  bore,  or  through  the  core  being  nearer  one  pole  piece 
than  the  other. 

This  may  result  from  unequal  wearing  of  the  bearings,  and  therefore 
the  bearings  should  either  be  relined  or  the  bolt  holes  of  the  bearings 
readjusted,  or  the  bearings  packed  up  until  the  armature  is  correctly 
centered. 

Oues.  What  happens  in  case  of  breaks  in  the  armature 
coils? 

Ans.  This  fault  results  in  local  heating  of  the  armature,  for 
the  reason  that  resistance  is  interposed  in  the  path  of  the  current 
at  the  fracture.  It  always  results  in  sparking  at  the  brushes, 
and  the  heating  being  confined  to  the  neighborhood  of  the  break. 

Oues.  What  are  the  effects  of  operation  above  the  rated 
voltage  and  below  normal  speed? 

Ans.  Voltage  above  normal  is  a  possible  cause  of  heating, 
and  operation  below  normal  speed  calls  for  an  increase  of  field 
strength  and  reduces  the  effective  ventilation,  thus  tending  to 
cause  heating. 


Gc.n 


HAWKINS  ELECTRICITY 


Fig.  730. — Forced  system  of  lubrication  as  applied  to  enxine  of  the  generatiiig  set  showc  in 
fig.  443.  In  engines  empirying  the  forced  system  of  lubncation  the  crank  pit.  which  is 
formed  by  the  columns,  is  accessible  through  doors  in  the  front  and  back  of  the  engine. 
The  base  of  the  engine  forms  an  oil  tank  to  which  is  attached  a  sniall  plunger  pump  dn\  en 
by  an  eccentric  on  the  shaft.  The  lubricant  is  carried  under  pressure  to  the  various  parts 
of  the  engine  by  the  mechanism  shown  in  the  accom.panying  diagram.  The  oil  is  forced 
by  a  pump  to  a  grcjve  in  the  main  bearing,  and  a  drilk-d  hole  in  the  shaft  connects  this 
groove  with  the  crank  pin.  Ftcrn  the  crank  i)in  box  the  oil  is  further  fcTced  to  the  wrist 
pin  through  the  pipe  Fanning  along  the  side  of  the  conncciing  red.  The  passage  in  the 
crosshead  allows  the  oil  to  be  foiced  from  the  wrist  pin  to  the  guides.  As  the  oil  is  forced 
from  one  bearing  to  another,  it  is  quite  important  that  the  bearing  caps  be  set  light, 
otherwise  the  oil  will  escape  before  reaching  the  last  bearing  After  passing  through  the 
bearings,  the  oil  is  collected  in  the  base,  strained  and  used  again.  The  oil  she  uld  be  free 
from  foreign  substances,  and  to  guar*'  against  the  introduction  of  any  foreign  matter, 
a  strainer,  which  may  be  taken  uat  for  examination  or  cleaning,  is  attached  to  the  suction 
valve  of  the  pump.  An  oil  pressure  of  from  10  to  20  lbs. should  be  maintained. and  may 
be  regulated  by  adjusting  the  set  screw  on  the  relief  valve  of  the  oiling  syslem.  The 
pressure  gauge  need  not  remain  in  the  circuit  continuously.  Only  mineral  oils  should 
be  used  for  lubrication.  A  heavy  oil  gives  better  results  and  prevents  knocking  more 
effectively  than  thin  oil.  An  nil  n-hich  has  been  found  to  give  good  resjlts,  consists  of 
two-thirds  red  engine  oil  and  one-third  heavy  cylinder  oil.  .As  the  oil  pas!«s  through 
the  bearings  repeatedly,  it  pradually  loses  its  lubricating  properties,  becoming  thick  and 
gritty,  and  should  be  occasi  irially  run  through  a  filter  and  mixed  with  new  oil.  The 
frequency  of  this  change  dep>c-nds  on  the  oil,  as  well  as  the  number  of  hours  the  engine  is 
in  operatioti,  and  can  easily  be  determine  by  observation.  The  oil  in  the  resci-voit 
should  stand  about  2  inch?s  over  the  suction  and  disch.irge  valves,  ai,d  no  water  should 
be  allowed  to  mix  with  it.  Should  any  "vater  accumulate  in  the  base-,  it  should  be  drawn 
off  by  the  cock  provided  for  the  purpose  before  starting  the  engine. 


HEATING  661 


Ques.     How  may  the  field  magnets  become  heated? 

Ans.  By  excessive  field  current;  eddy  current  in  pole  pieces; 
moisture;   short  circuits. 

Ques.  What  may  be  said  with  respect  to  excessive  field 
current? 

Ans.  When  heating  results  from  this  cause,  all  the  exciting 
coils  will  be  heated  equally.  It  may  be  due  to  excessive  voltage, 
in  the  case  of  shunt  dynam.os;  or  to  an  overload  in  the  case  of 
compound  and  series  dynamos.  In  either  case  it  may  be  remedied 
by  reducing  the  voltage  or  overload.  If  due  to  the  coils  being 
incorrectly  coupled  up,  that  is,  coupled  up  in  parallel  instead  of 
in  series,  it  will  be  necessary  to  rectify  the  connections  or  insert 
a  resistance  in  series. 

Ques.  State  the  causes  of  eddy  currents  in  the  pole 
pieces? 

Ans.  This  fault  may  be  due  to  defective  design  or  con- 
struction of  the  armature.  Slotted  armatures  are  particularly 
liable  to  cause  this  fault,  if  the  teeth  and  air  gap  be  not  properly 
])roportioned.  The  defect  may  also  be  occasioned  by  variation 
in  the  strength  of  the  exciting  current. 

If  due  to  this  latter  cause,  it  will  he  aecompauied  by  sparkinj;  at  the 
l)rushes.  If  a  shunt  dynamo,  insert  an  ammeter  into  the  shunt  eircuit, 
and  note  if  the  deflection  be  steady.  If  this  he  not  the  case,  the  variation 
in  the  current  most  probably  proceeds  from  imperfect  contacts  thrown 
into  vibration. 

Ques.     How  is  the  insulation  affected  by  moisture? 

Ans.  Moisture  tends  to  decrease  the  insulation  resistance, 
thus  in  effect  producing  a  short  circuit  with  its  attendant  heating. 

Ques.     How  is  moisture  in  the  field  coils  detected? 

An?:     It  \^  easily  detected  by  applying  the  hand  to  the  coils, 


662  HAWKINS  ELECTRICITY 

when  they  will  be  found  to  be  damp,  and  in  addition  steam  or 
vapor  will  be  given  off  where  the  machine  is  working. 

The  fault  may  be  remedied  by  drying  and  varnishing  the  coils. 

Ques.  What  is  the  indication  of  short  circuits  in  the 
field  coils? 

Ans.  This  fault  is  characterized  by  an  unequal  heating  of  the 
field  coils.  If  the  coils  be  connected  in  series,  the  faulty  coil  will 
be  heated  to  a  less  extent  than  the  perfect  coils ;  if  connected  in 
parallel,  the  faulty  coil  will  be  heated  to  a  greater  extent  than  the 
perfect  coils.  '  The  former  can  thus  be  easily  located. 


OPERATION  OF  MOTORS  663 


CHAPTER  XXXV 
.      OPERATION   OF   MOTORS 


In  operating  motors  of  any  considerable  size,  whether  con- 
nected to  the  public  supply  mains  of  a  central  generating  station 
for  combined  lighting  and  power  service,  or  to  power  service 
mains  only,  there  are  certain  precautions  to  be  observed  in 
starting,  stopping,  and  regulating  the  motor,  in  order  that  the 
efficiency  of  the  supply,  and  indirectly  the  working  of  other 
motors  and  lamps  connected  to  the  mains  in  the  immediate 
neighborhood,  may  not  be  affected  by  abnormal  variations  of 
prespiire.  These  precautions  should  be  observed  also  to  prevent 
any  danger  of  the  motor  itself  being  subjected  to  detrimental 
mechanical  shocks  and  excessive  temperatures  in  the  working 
parts. 

Before  Starting  a  Motor: — The  general  instructions  relat- 
ing to  inspection  and  adjustment,  lubrication,  etc.,  which  have 
already  been  given,  should  be  carefully  followed  preparatory 
to  starting. 

Starting  a  Motor. — In  starting  a  motor,  resistance  must  be 
put  in  scries  with  the  armature  because,  since  there  is  no  reverse 
electromotive  force  to  counteract  the  applied  voltage  when  the 
motor  is  at  rest,  the  switching  of  the  latter  direct  to  the  motor 
would  result  in  an  abnormal  rush  of  current.  This,  in  addition 
to  being  uneconomical  and  productive  of  a  drop  of  voltage  in 


664 


HAWKINS  ELECTRICITY 


the  mains,  would  injure  all  except  the  smallest  motors.     Hence 
motors  above  two  horse  power  usually  require  a  rheostat. 

Ques.     Describe  a  rheostat  or  "  starting  box." 

Ans.  It  consists  essentially  of  a  suitable  resistance  to  be 
inserted  at  starting  to  reduce  the  initial  rush  of  current,  and 
which  can  be  cut  out  in  sections  by  successive  movements  of  a 
lever  as  the  speed  increases. 


Jpffla 

X 

,, 

MiMi 

^» 

I 

iifli-F- 

COMMUTATOR 
3             ''■N  ^ 

ARMATURE 

(- 

z 

^ 

1 

.'/ 

Ji|il.l.WiH 

61EEVE   FOR  PRESSING  ON 


PIATE  Bjfl 


Fig.  731. — Press  for  forcing  on  and  removing  a  commutator.  Small  commutators  are  pressed 
on  to  the  shaft  by  a  hand  press.  All  of  the  larger  commutators  are  pressed  on  by  means 
of  a  power  press.  In  the  above  figure  is  shown  a  hand  press.  The  plate  B  is  used  in 
removing  old  commutators.  It  is  placed  back  of  the  commutator  as  at  a;  y  with  the  slot 
C  over  the  shaft.  Bolts  a  b  are  passed  through  the  holes  in  the  plate  and  secured  by 
nuts.  I'he  commutator  can  then  be  forced  off  the  shaft.  In  pressing  on  a  commutator,  a 
sleeve  is  placed  over  the  shaft  at  O,  and  against  the  commutator.  The  rear  end  of  the 
shaft  is  secured  so  it  will  withstand  the  pressure,  and  the  commutator  is  forced  on.  The 
power  presses  are  built  on  the  principle  of  a  hydraulic  press.  In  pressing  on  a  commuta- 
tor a  piece  of  babbitt  metal  or  soft  brass  should  be  used  against  the  end  of  the  shaft. 
The  shaft  should  be  painted  with  white  lead  before  having  the  commutator  pressed  on, 
in  order  to  lubricate  the  shaft  so  that  the  commutator  will  press  on  easily.  The  wiper 
rings  are  pressed  on  after  the  commutator  and  then  the  armature  is  ready  to  be  connected. 

Ques.     Describe  what  occurs  in  starting  a  motor. 

Ans.  When  the  lever  of  the  starting  box  is  moved  to  the  first 
contact  some  of  the  resistance  is  cut  out  of  the  circuit  and  current 
flows  through  the  motor.  This  produces  a  torque  and  starts  the 
armature  rotating.  The  movement  of  the  armature  induces  a 
reverse  voltage,  which,  as  the  speed  increases,  gradually  reduces 


OPERATION  OF  MOTORS 


665 


the  applied  ctirrent.  With  this  reduction  of  current,  the  torque 
is  reduced  and  the  speed  not  accelerated  as  quickly  as  at  first. 
"When  the  applied  current  has  been  reduced  to  a  certain  value  by 
the  increasing  reverse  current,  the  handle  of  the  starting  box  is 
moved  to  the  next  contact,  and  so  on  till  all  the  resistance  in  the 
starting  box  has  been  cut  out,  the  motor  then  attaining  its  normal 
speed. 


i|'- 


Figs.  732  to  735. — Various  starting  resistances.  The  type  of  resistance  used  in  motor  starting 
rheostats  of  small  size  consists  usually  of  tinned  iron  wire  wound  on  asbestos  tubes,  as 
shown  in  f\^.  732.  the  tubes  being  firmly  supported  by  porcelain  nipples,  the  ends  of  which 
fit  into  holes  in  the  top  and  bottom  of  the  enclosing  case.  In  starters  of  larger  size,  cast 
metal  grids,  as  shown  in  fig.  733,  are  used.  In  addition  to  these  types  of  resistance,  some 
forms  of  starter  are  equipped  with  what  is  known  as  "unit"  type  resistance.  In  this 
form,  the  resistance  is  built  up  of  a  number  of  separate  sections,  or  units,  which  are  con- 
nected to  form  the  complete  starting  or  regulating  resistance  as  the  case  may  be.  A  single 
unit  consists  of  a  moulded  core  of  \ntreous  material  upon  which  is  wound  the  resistance 
wire,  as  shown  in  fig.  734.  The  surface  of  the  unit  is  then  coated  with  a  special  cement  and 
baked.  By  this  method  the  resistance  material  is  protected  from  mechanical  injury  and 
is  also  made  proof  against  moisture  and  other  conditions  which  sometimes  affect  the 
ordinary  type  of  resistance.  In_addition  to  units  coated  with  cement  only,  there  are  still 
other  types  of  units,  as  in  fig.  735.  which  are  pro\'ided  with  a  sheet  metal  covering  around 
the  cement,  as  a  further  precaution  against  injury.  Each  of  the  various  types  of  resist- 
ance described  possesses  certain  characteristics  not  shared  by  the  others,  the  use  of  any 
particular  type  being  largely  governed  by  conditions  of  service. 


Oues.    What  is  the  difference  between  a  starting  box 
and  a  speed  regulator? 

Ans.     Motor   starting   rheostats    or    "  starting   boxes,"    are 


666 


HAWKINS  ELECTRICITY 


designed  to  start  a  motor  and  bring  it  gradtially  from  rest  to  full 
speed.    They  are  vot  intended  to  regulate  the  speed  and  must  nc 
be  used  for  such  purpose. 

FaiJure  to  obsenr  this  caution  will  result  in  burning  out  thf  resistance 
which,  in  a  motor  starter,  is  sufficient  to  carry  the  current  for  a  limited 
time  only,  whereas  in  the  case  of  speed  r^ulators  sufficient  resistance  is 
pro\-ided  to  carrj-  the  full  load  current  continuoush*. 


Pig.  736. — View  of  Cutler-Hammer  starter  with  slat€  front  ^elno^•ed  showing  open  wire  coil 
resistance.  The  type  of  resistance  here  used  consists  of  tinned  iron  wire  wound  on  ad>estos 
tubes.     The  bottom  of  the  casing  is  perforated  to  secure  \'entilation. 


Oues. 
used? 


For  what  kinds  of  service  are  speed  regulators 


Ans.     In  cases  when  the  speed  must  be  varied,  as  in  traction 
motors,  organ  blowers,  machine  tool  drive,  etc. 

Oues.     How  long  does  it  take  to  start  a  motor? 

Ans.     UsuaDy  from  five  to  ten  seconds. 


OPERATION  OF  MOTORS 


667 


Ques.     How  is  the  starting  lever  operated  ? 

Ans.  It  is  moved  progressively  from  contact  to  contact, 
pausing  long  enough  on  each  contact  for  the  motor  to  accelerate 
its  speed  before  passing  to  the  next. 

Ques.  What  are  the  conditions  at  starting  in  a  series 
motor  ? 

Ans.  There  is  a  rush  of  current,  the  magnitude  of  which 
depends  on  the  amount  of  resistance  cut  out  at  each  movement 
of  the  starting  lever. 


BUTTON  CONTACTS 


RENEWABLE 
CONTACTS 


Figs.  737  and  738. — Sliding  contact  starters.  Fig.  737,  starter  with  button  contacts;  fig.  738, 
starter  with  renewable  contacts.  Motor  starters  in  which  the  successive  steps  of  resistance 
are  cut  out  by  a  pivoted  lever  carrying  a  contact  shoe  which  slides  over  button  contacts 
or  over  contact  segments,  are  known  as  sliding  contact  starters.  Button  contacts  are 
usually  furnished  with  motor  starting  rheostats  of  small  size  while  contact  segments  are 
used  on  those  of  greater  capacity.  The  contact  segment  being  held  in  position  by  two 
screws,  is  readily  renewable  when  worn  by  long  service  or  damaged  by  arcing.  The  fixed 
button  contact  is  not  so  easily  renewed  but  being  used  only  on  small  size  starters  is  never 
likely  to  be  subjected  to  severe  service.  Some  starters,  however,  have  renewable  button 
contacts. 

Ques.  How  are  small  series  motors  started  on  battery 
circuits? 

Ans.  By  simply  closing  a  switch  to  complete  the  circuit,  the 
resistance  of  the  battery  being  sufficient  to  prevent  a  great  rush 
of  current  while  starting. 


668 


HAWKIXS  ELECTRICITY 


Oues.    How  is  a  shunt  motor  started? 

Ans.  In  starting  a  shtint  motor,  no  trouble  is  likely  to  occur 
in  connecting  the  field  coils  to  the  circuit.  Since  the  resistance 
of  the  armature  is  very  low,  it  is  necessary  on  constant  voltage 
circuits  to  use  a  starting  rheostat  in  series  with  the  armatiu-e. 

The  necessary  connections  are  shown  in  fig.  756.  The  switch  is  first 
closed  thus  sending  current  through  the  field  coils,  before  any  passes 
through  the  armature.    The  rheostat  lever  P  is  then  moved  to  the  first 


Figs.  739  and  740. — Multiple  switch  starters.  Fig.  739,  starter  with  no  voltage  release;  fig. 
740,  starter  with  no  voltage  release  and  circuit  breaker.  The  multiple  switch  type  of 
starter  is  designed  to  overcome  the  arcing  on  sliding  contacts  which,  in  the  case  of  large 
motors  would  be  very  severe.  The  cutting  out  of  each  step  of  resistance  is  accomplished 
in  the  multiple  switch  starter  by  a  separate  carbon  contact  switch  which  breaks  the  circuit 
with  a  quick  snappy  action. 


contact  to  allow  a  moderate  amount  of  current  to  pass  through  the 
armature.  The  resistance  of  the  rheostat  is  gradually  cut  out  by  further 
movement  of  the  lever  P,  thus  bringing  the  motor  up  to  speed. 

Oues.  How  does  the  reverse  voltage  affect  the  starting 
of  a  motor? 

Ans.  When  a  motor  is  standing  still,  there  is  no  reverse 
voltage,  and  the  ctirrent  taken  at  first  is  governed  principally 
by  the  resistance  of  the  circmt.  If  the  motor  be  series  wotmd, 
there  is  a  momentary  reverse  voltage,  due  to  self-induction  while 


OPERATION   OF  MOTORS 


669 


the  field  is  building  up.  If  the  motor  be  shunt  wound,  self- 
induction  delays  the  current  through  the  field  coils,  but  that 
through  the  armature  is  not  impeded  by  such  cause.  When  the 
armature  begins  to  revolve,  reverse  voltage  is  developed  which 
increases  with  the  speed.  The  resistance  of  the  starting  box 
may  be  gradually  cut  out  as  the  armature  comes  to  speed.  Thus 
the  reverse  voltage  gradually  replaces  ohmic  drop  in  limiting  the 
current  as  the  motor  comes  to  speed. 


Fig.  741.— Starting  rheostat  with  no  voltage  and  overload  release.  The  no  voltage  release 
permits  the  starting  lever  to  fly  to  the  "off  position"  should  the  voltage  fail  momentarily, 
thus  protecting  the  motor  against  damage  should  the  voltage  suddenly  return  to  the  line. 
The  movement  of  the  lever  is  due  to  a  spring.  The  overload  device  causes  the  lever  to 
back  to  the  off  position  should  the  current  exceed  a  predetermined  ma.ximum  for  wliich 
the  release  is  adjusted. 

Fig.  742. — Compound  starter.  Rheostats  designed  for  the  double  duty  of  starting  a  motor 
and  regulating  its  speed  are  commonly  known  as  compound  starters,  the  resistance  pro- 
\'ided  being  a  combination  of  armature  resistance  for  starting  duty  and  shunt  field  resistance 
for  speed  regulation. 


Failure  to  Start. — This  fault,  which  is  liable  to  occur  in  a 
motor  of  any  description,  is  similar  to  failure  to  excite  in  a  dy- 
namo, and  is  liable  to  be  produced  by  any  of  the  causes  mentioned 
in  connection  with  the  latter  fault,  excluding  insufficient  speed. 
and  insufficient  residual  magnetism. 

When  a  motor  fails  to  start,  it  should  first  be  ascer- 
tained  if   a   supply   of   electrical    energy  be  available  in   the 


670 


HAWKINS  ELECTRICITV 


mains.  This  may  readily  be  discovered  by  means  of  a  voltmeter, 
or  if  low  tension  service,  by  means  of  the  fingers  bridging  across 
the  main  terminals.  If  the  supply  of  energy  be  present,  the 
contact  arm  of  the  starter  should  be  moved  into  such  position 
that  all  resistance  is  inserted  into  circuit  with  the  motor.  This  is 
important,  as  the  motor  may  start  suddenly  while  trying  to 
ascertain  the  cause  of  the  stoppage. 


Fig.  743. — Starting  panel.  In  installing  any  kind  of  motor  starting  rheostat,  it  is  necessary 
to  proN-ide  main  line  knife  switch  and  luses  in  addition  to  the  starting  box.  The  appearance 
of  the  installation  can  be  much  improved  by  mounting  all  of  these  upon  one  panel. 


Having  closed  the  switch,  if  the  motor  fail  to  start,  it  will  be 
advisable  to  remove  the  load  if  possible,  as  the  failure  may  arise 
from  an  overload  of  the  machine.  This  being  effected  and  the 
motor  not  starting,  the  terminals  of  the  latter  should  be  tested 
by  the  means  already  described  for  voltage.     If  no  voltage  be 


OPERATION  OF  MOTORS 


671 


generated,  a  broken  circuit  or  a  defective  contact  may  be  looked 
for  in  the  main  fuse,  switch,  or  starting  box.  The  resistance 
coils  of  the  latter,  through  the  heat  developed,  frequently  break 
in  positions  out  of  sight.  If  a  defective  contact  of  this  nature 
cannot  readily  be  seen,  the  contact  arm  should  be  moved  slowly 
over  the  contacts,  as  it  is  possible  the  broken  coil  may  be  cut  out 
of  circuit  by  this  means. 

If  a  difference  of  pressure  exist  between  the  motor  terminals,  the  field 
magnets  will,  if  shunt  or  compound  wound  and  in  good  order,  be  excited, 
which  may  be  ascertained  by  means  of  a  bar  of  iron.    If  no  magnetism 


Figs.  744  to  746. — Cutler-Hammer  motor  starting  rheostats  with  no  voUage  and  overload 
release.  Fig.  744,  starter  with  fixed  button  contact,  fig.  74.T,  with  renewable  button  contact, 
and  fig.  74G,  with  contact  segments.  In  construction  the  resistance  is  enclosed  in  a  pressed 
steel  box  on  which  is  mounted  a  marbleized  slate  panel  carrying  the  starting  lever,  contacts 
and  protective  devices.  By  means  of  a  calibrated  scale,  the  overload  release  (shown  in  the 
lower  left  hand  comer,  figs.  744  and  74.5,  and  in  the  lower  right  hand  comer  fig.  746)  can 
be  set  to  break  the  circuit  on  any  overload  not  exceeding  .50  per  cent,  of  the  rated  capacity 
of  the  motor.  This  calibrated  scale  can  also  be  used  for  determining,  with  a  fair  degree  of 
accuracy,  the  amount  of  current  being  consumed  by  the  motor. 


be  present,  it  will  of  course,  indicate  a  broken  or  bad  connection,  either 
between  the  terminals  of  the  field  coils,  or  one  or  more  of  the  coils  them- 
selves. If  the  bar  pull  strongly,  the  position  of  the  brushes  upon  the 
commutator  in  regard  to  the  neutral  points  should  be  ascertained,  and 
the  rocker  adjusted,  if  necessary,  to  bring  them  into  their  correct  po.si- 
tions.  If  this  fail  to  start  the  motor,  the  connecting  leads  from  the  motor 
terminals  to  the  brushes  and  the  brushes  themselves  should  be  carefully 
examined  for  broken  or  bad  connections,  and  defective  contact  of  tbi 


672 


HAWKINS  ELECTRICITY 


brushes  wilh  the  commutator.  In  the  latter  case,  it  may  arise  from  a 
dirty  state  of  the  commutator,  or  from  the  brushes  not  being  fed  properly. 
If  due  to  these  causes,  pressing  the  brushes  down  upon  the  commutator 
with  the  fingers  will  probably  start  the  motor.  If  the  failure  to  start 
arise  from  none  of  these  causes,  it  is  probably  due  to  the  field  coils 
acting  in  opposition,  or  to  a  short  circuited  armature.  This  latter 
remark  applies  more  especially  to  motors  provided  with  drum  armatures. 


Fig.  747. — Allen-Bradley  compression  type  resistance  units.  The  contact  resistance  between  the 
discs  composing  the  resistance  column  is  suliject  to  variations  of  pressure,  thereby  producing 
proportionate  resistance  changes  in  the  column  as  a  whole.  In  the  complete  resistance 
unit,  the  resistances  column  is  encased  in  a  drawn  steel  tube,which  is  lined  with  a  highly 
refractory  cement,  for  purpose  of  insulation,  fl/TorJjjz!,' ^/;«  column  both  mcchanicdl  and  elec- 
trical protection  and  excluding  the  air  which  effectually  prevents  any  combustion  should 
the  column  l:>ecome  red  hot  due  to  overload.  The  ends  of  the  tube  are  closed  by  means 
of  caps  through  which  pass  electrodes  for  making  connections  between  the  discs  and  exterior 
conductors.  The  steel  tube,  when  necessary,  is  provided  with  ribs  or  fins  for  the  dissipation 
of  acquired  heat. 


Precautions  with  Shunt  Motors. — With  motors  of  this  type, 
because  of  the  large  amount  of  self-induction  in  the  shunt  wind- 
ings, it  is  important  to  note:  1,  that  in  switching  on  the  field 
magnet,  the  current  may  take  an  appreciable  time  to  grow  to  its 


OPERATION  OF  MOTORS 


673 


PiG.  748. — Allen-Bradley  type  Z  automatic  motor  starter.  The  operation  of  this  machine  is 
as  follows:  When  the  main  switch  is  closed,  the  motor  circuit  is  made  through  the  fuses, 
resistance  unit,  current  relay,  and  the  motor  armature.  At  the  same  time,  the  solenoid 
circuit  is  closed  (this  is  connected  directly  across  the  line,  and  takes  a  current  which 
is  a  small  fraction  of  an  ampere),  and  the  plunger  of  the  solenoid  is  drawn  up,  which 
produces  a  pressure  on  the  resistance  unit,  and  increases  the  current  in  the  motor  circuit 
to  the  predetermined  value  at  which  the  current  relay  is  set.  When  this  value  is  reached 
the  current  relay  operates  and  opens  the  solenoid  circuit,  which  reduces  the  magnetic  pull 
and  allows  the  solenoid  plunger  to  drop  back  slightly.  This  action  increases  the  resistance 
in  the  motor  circuit,  which  decreases  the  current  sufficiently  to  allow  the  relay  to  close 
again.  Similar  cycles  of  operation  are  repeated  as  the  motor  accelerates,  and  each  time 
the  plunger  is  drawn  a  little  farther  into  the  solenoid,  until  the  short  circuiting  contacts  on 
the  top  are  pushed  together,  which  short  circuits  the  current  relay  and  resistance  unit, 
making  them  inoperative,  and  completing  the  operation  of  starting  the  motor.  It  will 
be  noted  that  in  starting  a  motor  with  this  device  the  current  is  always  held  down  to  a 
certain  predetermined  value,  and  it  is  impossible  to  overload  the  motor  by  too  rapid 
starting.  The  current  relay  is  calibrated  in  amperes,  and  may  be  set  to  suit  existing 
conditions.  The  action  of  the  starter  being  controlled  by  a  current  relay  and  not  by  an 
oil  or  air  dash  pot.  the  motor  will  start  rapidly  when  under  a  light  load,  and  slowly  when 
more  heavily  loaded.  The  fuses  or  circuit  breakers  may  be  set  at  a  value  that  will  furnish 
protection  to  the  motor  upder  running  conditions. 


674 


HAWKINS  ELECTRICITY 


normal  value,  and  2,  that  in  s\\ntching  off,  especially  wnth  quick 
break  swntches,  high  voltages  are  induced  in  the  VN-indings,  which 
may  break  down  the  insulation. 


Fig.  749. — Monitor  starter  giving  automatic  start  with  knife  switch  control;  designed  for  use 
with  printing  presses.  It  consists  of  a  set  of  solenoids  connected  in  series  and  so  inter- 
operating  as  to  cut  reastance  out  of  the  armature  circuit  of  the  motor  as  the  apparatus 
it  is  dri\-in(j  comes  up  to  speed.  This  tj-pe  is  for  small  motors  or  where  no  need  arises 
for  speed  regulation;  there  is.  therefore,  no  adjustment  of  speed  possible  aside  from 
an  actual  chajige  in  motor  conditions.  At  full  speed  the  motor  is  directly  across  the  main 
supply  line. 

Fig.  750. — Monitor  automatic  starter,  equipped  with  relay  for  push  button  control. 


Oues.  What  provision  is  made  so  that  the  magnetizing 
current  will  have  time  to  reach  its  normal  value? 

Ans.  The  field  connections  are  generally  separated  from  the 
actual  starter,  and  taken  to  the  main  switch,  so  that  wherever 
the  main  switch  is  closed,  the  current  flows  through  the  field 
coils,  before  the  starting  lever  is  moved. 


OPERATION  OF  MOTORS 


675 


Ques.     How  are   the   connections   arranged    to   avoid 
excessive  voltage  in  the  windings  due  to  self-induction? 

Ans.     Generally  the  armature  and  field  magnet  circuits  are 
placed  in  a  closed  circuit  that  is  never  opened. 


In  other  cases,  in  order  that  the  rise  of  voltage  may  not  injure  the 
insulation  when  the  shunt  is  opened,  a  special  form  of  main  switch  is 
sometimes  used  which,  before  breaking  from  the  suppl}',  puts  a  non- 
inductive  resistance  across  the  shunt  of  the  motor.  This  is  known  as  a 
flashing  resistance. 


Pigs.  751  to  753. — Monitor  control  switches.  Fig.  751,  push  button  "start"  and  "stop" 
switch;  fig.  752.  safety  lever  control  switch  with  "slow"  and  "fast"  buttons  for  rotary- 
printing  presses.  This  device  will  upon  pressure  of  the  "start"  button,  set  the  machine 
in  motion  and  bring  it  up  to  the  predetermined  speed,  either  as  previously  set  by  the 
starter  limits  or  by  the  setting  of  the  rheostat  arm.  The  stop  button  projects  some  dis- 
tance beyond  any  other  portion  of  the  device,  in  order  that  in  case  of  emergency  the 
operator  may  stop  the  machine  merely  by  hitting  the  face  of  the  switch  with  his  open 
hand.  The  lever  control  switch,  fig.  753,  is  similar  in  its  action  to  the  push  button  switch 
but  combines  two  other  features:  locking  point,  and  visual  indication  of  the  station  from 
whence  the  press  has  been  stopped.  With  the  lever  at  the  downward  position,  the  press 
is  locked  and  cannot  be  started  from  any  other  station.  In  order  to  make  the  press  ready 
to  start  the  lever  must  be  raised  to  the  central  position.  Thus  a  rnan  may  safely  enter 
the  press  without  delay  by  setting  his  station  to  the  locked  position,  knowing  that  it 
cannot  be  started  except  by  some  one  coming  to  that  station  and  realizing  that  the  press 
has  been  purposely  locked.  Also,  by  looking  along  the  press,  it  is  possible  to  tell  from 
which  station  it  has  been  locked,  and  proper  action  can  be  immediately  taken.  The  safety 
control  station  is  usually  combined  for  use  on  large  rotary  presses  with  the  "slow"  and 
"fast "  push  buttons  as  shown  in  fig.  752.  A  pressure  upon  the  fast  or  slow  buttons  will 
cause  the  press  speed  to  be  correspondingly  accelerated  or  retarded,  and  this  action  will 
continue  so  long  as  the  button  is  pressed.  The  press  continues  to  run  at  the  speed 
attained  at  the  instant  of  releasing  the  button.  Any  speed  may,  therefore,  be  selected  or 
changed  to  suit  momentary  requiremente-  This  gives  complete  control  excepting  reversal 
which  is  not  required  of  such  a  pras& 


676 


HAWKINS  ELECTRICITY 


Fig.  754. — Wiring  diagram  of  the  standard  fonn  of  Monitor  controller.  A  set  of  solenoids  are 
connected  in  series  and  so  interoperating  as  to  cut  resistance  out  of  the  armature  circuit  of 
the  motor  as  the  apparatus  it  is  dri\-ing  comes  up  to  speed.  The  controller  is  designed  to 
be  used  on  all  classes  of  work.  In  its  simplest  form,  a  single  copper  and  graphite  contact, 
is  controlled  by  two  magnets,  so  proportioned  as  to  cut  out  the  entire  starting  resistance 
when  the  armature  current  falls  to  a  predetermined  value.  In  the  larger  sizes,  the  number 
of  steps  controlling  the  resistance  is  increased  and  arranged  to  produce  the  correct  acceler- 
ation. In  every  case  the  regulation  of  the  starting  resistance  is  effected  entirely  by  the 
current  passing  to  the  motor  without  the  use  of  dash  pot,  mechanical  governor  or  delicat« 
cut  outs.  The  time  element  is  thus  directly  proportioned  to  the  load  and  the  motor  brought 
up  to  speed  in  the  shortest  time  consistent  with  the  load,. but  always  with  safe  limitation 
of  the  armature  current.  The  distinction  between  the  current  controlled  starter  and  the 
starter  with  dash  pot  governor  should  be  noted.  The  starter  here  shown  limits  the  start- 
ing current  to  a  fixed  value  throughout  the  Starting  operatioa.  which  is  an  ideal  oandition 
and  prevents  blowing  ftises  in  starting. 


OPERATION  OF  MOTORS  677 

Oues.  How  can  shunt  motors  be  controlled  from  a 
distant  point? 

Ans.  The  starter  and  switch  are  placed  at  the  desired  point 
and  the  two  main  wires  and  the  field  wires  run  from  that  point 
to  the  motor. 

This  requires  additional  wire  which  increases  the  cost  and  Hne  loss. 

Regulation  of  Motor  Speed. — Motors  are  generally  run  on 
constant  voltage  circuits.  Under  these  conditions,  the  speed 
of  series  motors  varies  with  the  load  and  at  light  loads  becomes 
excessive.    Shunt  motors  run  at  nearly  constant  speeds. 

For  many  purposes,  particularly  for  traction,  and  for  driving 
tools,  it  is  desirable  to  have  speed  regulation,  so  that  motors 
running  on  constant  voltage  circuits  may  be  made  to  run  at 
different  speeds. 

The  following  two  methods  are  generally  used  for  regulating 
the  speed  of  motors  operated  on  constant  voltage  circuits: 

1.  By  inserting  resistance  in  the  armature  circuit  of  a  shunt 
wound  motor; 

2.  By  varying  the  field  strength  of  series  motors  by  switch- 
ing sections  of  the  field  coils  in  or  out  of  circuito 

Oues.     Describe  the  first  method. 

Ans.  This  method  is  illustrated  in  fig.  756.  When  the  main 
switch  is  closed,  the  field  becomes  excited,  then  by  moving  the 
lever  P  of  the  starting  rheostat  the  various  contacts  (1,  2,  3,  4,  5), 
more  or  less  of  the  rheostat  resistance  is  cut  out  of  the  armature 
circuit,  thus  varying  the  speed  correspondingly. 

This  is  the  same  as  the  method  of  starting  a  motor,  that  is,  by  variation 
of  resistance  in  armature  circuit,  but  it  should  be  noted  that  when  this 
method  is  used  for  speed  regulation,  a  speed  regulating  rheostat  should 
be  used  instead  of  the  ordinary  starting  box,  because  the  latter,  not  being 
designed  for  the  purpose,  will  overheat  and  probably  burn  out. 


67  ^ 


UA  WKINS  ELECTRICITY 


Fig.  755. — Monitor  printing  press  controller.     It  provides  var: 

features  required  in  the  operation  of  large  rotarj,-  presses,  such  ; 
newspapers.     From  any  one  of  various  stations'  amilar  to  the  c 
located  at  all  desirable  places  about  the  press,  the  latter  mj;.-  'b^e 
ated,  slowed  down  or  locked.     It  differs  from  other  i>-pei 
that  the  solenoid  has  an  overall  marimum  pull  of  less  i'hazi 
the  main  line  current  directly  but  through  pilot  circuits,  ■ 
switches;  there  are  no  sliding  contacts.     At  the  control  sta::-?, 
guish  the  accelerating  button  from  the  retarding  button  by  the 
ously  he  can  in  the  same  manner  ascertain  the  f^:  ?■••  ;n    f  the  ';■ 
■"ever  whether  at  start,  stop  or  safety,  can  be  r 
lever  of  either  control  station  is  placed  at  stop.  - 
and  a  powerful  d>Tiamic  brake  brings  the  pre> 
or  harmful  strain.     The  start  will  always  be  • 
circuit,  and  with  fuU  field,  and  should  the  cu— 
and  open  the  circuit  to  the  motor.     This  contr: 
of  normal  speed  by  annatare  resistance  and.  by  ztli  co:::r;:, 
speed  of  the  motor. 


OPERATION  OF  MOTORS 


679 


Ques.    Describe  the  second  method. 

Ans.  This  niclliod  of  regulating  the  speed  of  a  series  motor 
is  shown  in  fig.  757.  The  current  through  the  armature  will 
flow  through  all  the  field  windings  when  the  position  of  the  switch 
lever  S,  is  on  contact  4,  and  the  strength  of  the  field  will  be  the 
maximum.  By  moving  the  arm  to  contact  3,  2  etc.,  sections  of 
the  field  winding  are  cut  out,  thus  reducing  the  strength  of  field 
and  varying  the  speed. 


OM^ 


Fig.  756. — Spead  regulation  of  shunt  motor  by  variable  resistance  in  the  armature  circuit. 


Ques.     How  does  the  speed  vary  with  respect  to  variation 
of  field  strength  ? 

Ans.     Decreasing  the  field  strength  of  a  motor  increases  its 
speed,  while  increasing  the  field  strength  decreases  the  speed. 

Under  the  conditions  of  maximum  field  strength,  as  with  switch  S  on 
point  1,  the  torque  will  be  greatest  for  any  given  current  strength  and 
the  reverse  voltage  also  greatest  at  any  given  speed.  The  current  through 
the  armature  of  the  motor,  to  perform  any  given  work,  will  thus  be  a 


C80 


HA  WKIXS  ELECTRICITY 


Pig.  757. — Speed  regolatiaa  of  series  motor  by  cntthig  oat  sections  of  the  field  winding, 
bi  this  method  the  fidd  winding  is  tai>ped  at  several  points,  divi^ng  the  oofl  into  sections 
and  the  leads  from  these  points  are  connected  a  nnilti-point  switdi  of  the  type  that  woold 
be  nsed  on  a  rheostat.  By  moving  the  lever  S,  to  the  I^  or  right,  the  corrent  will  flow 
throng  one  or  more  sections  of  the  field  winding,  thus  decreasing  or  increasing  the  ampere 
turns  and  therdiy  providincr  means  of  regulation. 

NOTE. — .\  compound  motor  may  be  made  to  run  at  constant  speed,  if  the  current  in 
the  series  winding  of  the  field  be  arranged  to  act  in  opposition  to  that  of  the  shunt  winding. 
In  such  case,  an  increase  of  Ic^id  will  weaken  the  fields  and  allow  more  current  to  flow  through 
the  armature  without  decreasicg  the  speed  of  the  armature,  as  would  be  necessary  in  a  shunt 
motor.  Such  motors,  however,  are  not  very  c^en  used,  since  an  overload  would  weaken  the 
fields  too  much  and  cause  trouble.  If  the  current  in  the  series  field  act  in  the  same  direction 
as  that  in  the  shunt  fields,  the  motor  will  slow  up  some  when  a  heavy  load  comes  on,  but  will 
take  care  of  the  load  without  much  trouble. 

NOTE. — Motors  have  much  the  same  faults  as  dynamos,  but  they  make  themselves 
manifest  in  a  different  way.  An  open  field  circuit  will  prevent  the  motor  starting,  and  will 
cause  the  melting  of  fuses  or  burning  out  of  the  armature.  A  short  circuit  in  the  fields,  if 
it  cut  out  only  a  part  of  the  winding,  will  cause  the  motor  to  run  faster  and  very  likely  spark 
badlv.  If  the  brumes  be  not  set  exactly  opposite  each  other,  there  will  also  be  bad  sparking. 
If  they  be  not  at  the  neutral  point,  the  motor  will  spark  badly.  Brushes  should  always  be  set 
at  the  point  of  least  sparldng.  If  it  become  necessary  to  open  the  field  circuit,  it  should  be 
done  slowly,  letting  the  arc  gradually  die  out.  A  quick  break  of  a  circuit  in  connection  -xith 
any  dynamo,  or  motor  is  not  advisable,  as  it  is  very  likely  to  break  down  the  insulation  of  the 
machine.  The  ordinary  starting  box  for  motors  is  wound  with  comparatively  fine  wire  and 
will  get  very  hot  if  left  in  circuit  long.  The  movement  of  the  arm  from  the  first  to  the  last 
point  should  not  occupy  more  than  tUrty  seconds  and  if  the  armature  do  not  begin  to  move 
at  the  first  point,  the  ann  should  be  thrown  back  and  the  trooble  located. 


OPERATION  OF  MOTORS 


681 


minimum,  as  well  as  the  speed  at  which  the  motor  has  to  run,  in  order  to 
develop  sufficient  reverse  voltage  to  permit  this  current  to  flow.  Regu- 
lation of  speed  by  varying  the  field  strength  is  limited  in  range  of  action, 
since  the  field  saturation  point  is  soon  reached,  moreover,  with  too  low 
a  field  strength,  armature  reaction  produces  excessive  field  distortion, 
sparking,  etc. 


Tj  Earth 


/iG.  758. — Speed  regulation  of  a  series  motor  by  the  method  of  short  circuiting  sections  of  the 
field  winding.  It  will  be  seen  that  there  are  seven  different  positions  for  the  contact 
springs  on  the  barrel  contacts.  A.  represents  the  armature  and  brushes,  little  A,  B,  and  C. 
the  divided  field  magnet  coils,  L  the  line  connection,  and  G  the  earth  connection.  The 
diagram  shows  the  connections  for  trolley  car  operation. 


Oues.  How  is  the  speed  of  shunt  and  compound 
motors  varied  with  respect  to  the  normal  speed  in  the  two 
methods? 

Ans.  The  first  method  (variable  resistance  in  armature 
circuit)  reduces  the  speed  below  the  normal  or  rated  speed  of  the 
machine,  while  the  second  method  increases  the  speed  above  the 
normal. 


In  the  first  method  the  amount  of  speed  reduction  depends  partly 
upon  the  amount  of  resistance  introduced  into  the  armature  circuit,  and 
partly  upon  the  load. 

In  the  second  method  the  amount  of  speed  increase  depends  entirely 
upon  the  amount  of  resistance  placed  in  the  shunt  winding  circuit. 


682  HAWKINS  ELECTRICITY 

Eighty-five  per  cent,  is  about  the  maximum  speed  reduction 
obtainable  by  armature  resistance  but  so  great  a  reduction  is 
seldom  satisfactory  since  comparatively  slight  increases  in  the 
load  will  cause  the  motor  to  stall. 

Shunt  field  regulation  may  be  obtained  up  to  any  point  for  which  the 
motor  is  suited,  the  only  limitation  in  this  case  being  the  maximum 
speed  at  which  the  motor  may  be  safely  operated. 

It  should  be  remembered,  however,  that  speed  increase  by  shunt 
field  weakening  increases  the  current  in  proportion  to  the  increase  in 
speed,  and  care  should  be  taken  not  to  overload  the  armature. 


Fig.  759. — Cutler-Hammer  multiple  switch  starter  with  no  voltage  release;  for  use  with  large 
motors,  or  with  motors  of  medium  size  where  the  starting  conditions  are  severe  or  when 
more  than  fifteen  seconds  are  required  to  accelerate  the  motor.  In  operation,  the  cutting 
out  of  each  step  of  resistance  is  accomplished  by  a  separate  lever  and  the  levers  them- 
selves are  so  interlocked  as  to  prevent  closing  switches  except  in  proper  order,  beginning 
with  the  lever  on  the  left.  The  last  switch  (the  one  on  right  hand  side)  is  held  by  an  electro- 
magnet w^hen  closed,  each  of  the  other  switches  being  held  in  the  closed  position  by  a 
latching  device  on  the  switch  next  to  it.  In  front  of  each  switch  is  placed  a  metal  stop,  so 
arranged  as  to  prevent  any  switch  being  operated  tmtil  the  one  next  to  it  on  the  left  has 
been  closed.  These  metal  stops  constitute  the  interlocking  mechanism  and  prevent  the 
starting  of  the  motor  in  any  way  except  by  closing  the  switches  in  regular  rotation,  thus 
insuring  proper  resistance  in  the  circuit  and  protecting  the  motor  from  excessive  starting 
currents.  When  the  current  is  interrupted,  the  electro-magnet  releases  the  last  switcli, 
which,  on  opening,  releases  the  latch  on  the  switch  next  to  it,  allowing  that  switch  to  open, 
and  this  in  turn  releases  the  next  latch  and  so  on,  the  switches  opening  automatically  one 
after  another.  In  starting  the  motor,  each  switch  should  be  closed  quickly  and  firmly, 
pausing  a  second  or  two  before  closing  the  next  switch  to  give  the  motor  time  to  accelerate. 

N'OTE. — In  starting  a  motor,  first  see  that  the  bearings  contain  sufficient  oil  and  that  the 
brushes  bear  evenly  on  the  commutator.  If  a  circuit  breaker  be  used,  close  it;  then  close  the 
main  switch.  Rotate  slowly  the  handle  of  the  starting  rheostat  as  far  as  it  will  go.  Care 
should  be  taken,  in  starting  the  motor,  that  the  handle  of  the  rheostat  be  not  rotated  too  fast. 
To  stop  a  motor,  open  the  circuit  breaker  or  switch,  which  wall  cut  in  the  resistance  of  the 
starting  box.  Never  attempt  to  stop  a  motor  by  forcibly  pulling  open  the  starting  box. 
Disregard  of  these  instructions  may  cause  burning  out  of  the  field  coils. 


OPERATION  OF  MOTORS 


683 


Ques.     How  is  a  wide  range  of  speed  regulation  secured? 

Ans.     By  a  combination  of  the  two  methods. 


Regulation  by  Armature  Resistance. — Speed  regulators 
for  this  method  of  regulation,  are  designed  to  carry  the  normal 
current  on  any  contact  without  overheating  and  when  all  the 
resistance  is  in  the  circuit,  they  will  reduce  the  speed  of  the  motor 


Fig.  700. — Cutler-Hammer  speed  regulator  with  no  voltage  release,  regulation  by  armature 
resistance  only,  reducing  speed  of  motor  below  normal.  No  resistance  in  the  armature  cir- 
cuit. No  provision  is  made  in  regulators  of  this  type  for  increasing  the  speed  of  the  motor. 
The  maximum  speed  obtamable  when  these  regulators  are  used  is,  therefore,  the  normal 
speed  at  which  the  motor  is  designed  to  operate  with  no  resistance  in  circuit.  With  all 
resistance  in  circuit  and  the  motor  taking  normal  current  these  regulators  will  reduce  the 
speed  of  the  motor  .50  per  cent.  If  the  motor  be  taking  less  than  normal  current  the  per- 
centage of  speed  reduction  obtainable  will  be  correspondingly  less.  The  notched  fan  tail 
extension  on  the  lower  end  of  the  lever  engages  with  a  magnetically  operated  pawl  to  hold 
the  lever  squarely  on  any  contact  so  long  as  the  no  voltage  release  magnet  is  energized. 


about  50  per  cent,  provided  the  motor  be  taking  the  normal 
current.  When  operating  without  resistance  in  the  armature 
circuit,  shunt  wound  and  compound  wound  motors  will  regulate 
to   approximately   constant   speed   regardless   of   load.     This 


684 


HAWKINS  ELECTRICITY 


characr eristic  of  inherent  regtilation  is  lost,  however,  when 
armature  resistance  is  employed  to  reduce  the  speed  of  the  motor, 
fluctuations  in  load  resulting  in  fluctuations  in  speed,  which 
become  more  noticeable  as  the  amount  of  resistance  inserted 
in  the  armature  circuit  is  increased.  Accordingly,  it  becomes 
necessary  to  move  the  lever  of  the  speed  regulator  forward  or 
backward  to  again  obtain  the  speed  at  which  the  machine  was 
operating  before  the  load  changed. 


Pig.  761. — Cntler-Haimner  compound  starter  with  no  voltage  and  overload  release.  This  is 
a  staiting  riieostat  and  field  regulator  combined.  In  operation,  two  levers  are  employed, 
both  being  mounted  on  the  same  hub  i>ost  and  one  lying  directly  under  the  other.  The 
upper  lever  only  is  pro\"ided  with  a  handle,  but  when  moving  from  the  off  position  to  the 
starting  position  (that  is  to  say,  from  left  to  right)  the  lower,  or  starting,  lever  is  carried 
along  by  the  upper,  or  speed  regiilating,  lever  until  it  comes  in  contact  with  the  no  voltage 
release  magnet  where  it  is  held  fast  by  the  attraction  of  the  magnet,  lea\'ing  the  upper 
lever  free  to  be  njoved  backward  over  the  field  contacts,  thus  weakening  the  shunt  field  of 
the  motor  little  by  little  until  the  desired  speed  is  attadned.  During  the  operation  of  start- 
ing the  motor,  the  f.ejd  resistance  is  short  circuited  by  an  auxiliary  contact  (the  slotted 
metal  strip  shown  near  center  of  rheostat)  but  as  soon  as  the  starting  lever  touches  the  no 
voltage  release  magnet  or.  In  other  words,  when  the  motor  has  been  accelerated  to  normal 
speed,  this  short  circuit  is  removed,  and  the  field  resistance  becomes  effective  for  speed 
regulation.  The  motor  is  accelerated  from  rest  to  normal  speed  by  moving  both  levers 
from  left  to  right,  while  increases  in  speed  above  normal  are  obtained  by  moving  the  upper 
lever  from  right  to  left.  Only  the  lower,  or  starting  lever  comes  into  contact  with  the 
no  voltage  release  magnet.  This  lever  ispro\-ided  with  a  strong  spiral  spring  which  tends 
always  to  throw  the  lever  back  to  the  off-position.  Hence  should  the  voltage  fail,  the  no 
voltage  release  magnet  releases  the  starting  lever  and  this,  in  flying  back  to  the  off  position, 
opens  the  armature  circuit  of  the  motor  and  carries  the  speed  regulating  lever  with  it  to 
the  off  position.  The  upper,  or  speed  regulating  le\'er,  not  being  influenced  by  the  spring, 
though  mounted  on  the  same  hub  post  as  the  starting  lever,  may  be  mov^  back  and  for^ 
»t  will,  or  left  vndefiajt«ly  ia  the  position  which  gives  the  spe«4  desired.    "  '  ' 


OPERATION  OF  MOTORS  685 


When  the  speed  of  a  motor  driving  a  constant  torque  machine  is 
reduced  by  inserting  resistance  in  the  armature  circuit  there  is  no 
corresponding  reduction  in  current  consumed.  The  motor  runs  more 
sloA^ly  simply  because  a  part  of  the  energy  impelling  it  is  shunted  into 
the  resistance  and  there  dissipated  in  the  form  of  heat.  Hence,  whether 
the  motor  be  operatin*^  at  fall  SDced  or  half  speed,  the  amount  of  current 
consumed  is  the  same;  the  only  difference  being  that  in  the  one  case 
all  the  energy  taken  from  the  line  is  expended  in  driving  the  motor  while 
in  the  other  case  only  one  half  is  utilized  for  power,  the  other  half  being 
dissipated  in  the  resistance.  Speed  regulation  by  armature  resistance 
only  is  therefore  open  to  two  objections:  1,  the  difficulty  of  maintaining 
constant  speed  under  varying  load  conditions,  and  2,  the  necessity  of 
wasting  energy  to  secure  speed  reduction.    These  objections  are,  in  part, 


Fig.  762. — Cutler-Hammer  compound  speed  regulator  with  no  voltage  and  overload  release; 
regulation  by  combined  armature  and  shunt  field  resistance,  designed  to  both  decrease  and 
increase  the  speed  of  a  motor.  Speed  reduction  is  accomplished  by  inserting  resistance  in 
the  armature  circuit,  the  maximum  amount  of  speed  reduction  obtainable  with  these  con- 
trollers being  .50  per  cent,  below  normal.  Speed  increase  is  obtained  by  inserting  resistance 
in  the  shunt  field  circuit,  the  maximum  amount  of  speed  increase  obtainable  with  these 
controllers  being  25  per  cent,  above  normal. 

offset  by  the  fact  that  speed  reduction  by  armature  resistance  may  be 
applied  to  any  motor  of  standard  design  and  requires  nothing  more  than 
the  simplest  and  least  expensive  speed  regulating  rheostat. 

In  cases  where  the  motor  will  be  operated  nearly  always  at  full  speed, 
the  difference  in  first  cost  of  the  installation  may  justify  the  use  of  the 
armature  resistance  method  of  control.  As  a  rule,  speed  regulation  by 
shunt  field  resistance  is  preferable. 

Regulation  by  Shunt  Field  Resistance. — Since  regulation 
by  his  method  is  for  speeds  above  normal,  a  starter  must  be  used 
to  bring  the  motor  up  to  its  rated  speed.    Usually  the  starter  is 


686 


HAWKINS  ELECTRICITY 


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OPERATION  OF  MOTORS  687 


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688 


//.4]rA'/.V5  ELECTRICITY 


OPERATION  OF  MOTORS 


689 


combined  with  the  regulator,  as  shown  in  fig.  761,  the  device 
being  called  a  compound  starter. 

The  weakening  of  the  shunt  field  of  a  motor  by  the  insertion 
of  resistance  in  the  shunt  field  circuit  causes  the  armature  to 
revolve  more  rapidly.    One  advantage  of  this  method  of  control 


Fig.  707. — Cutler-Hammer  reversible  starter  with  no  voltage  release,  adapted  to  start  and 
operate  motor  at  full  speed  in  either  direction,  such  for  instance  as  motors  driving  auxiliary 
motions  on  lathes,  planers  and  other  machine  tools  which  may  rotate  in  either  direction 
but  always  at  constant  speed.  They  are  not  designed  to  reduce  the  speed  of  the  motor, 
but  merely  to  start  it  and  bring  it  smoothly  up  to  full  speed  in  either  direction.  Two  no 
voltage  release  latching  devices  are  provided  so  that  the  lever  will  be  held  in  the  full  speed 
position  in  either  direction  so  long  as  the  voltage  of  the  line  remains  constant.  On  failure 
of  voltage  a  strong  centering  spring  attached  to  the  hub-post  of  the  lever  throws  the  latter 
to  the  central,  or  off  position.    The  shunt  field  circuit  is  not  opened  by  starters  of  this  type. 


is  that  the  motor  will  inherently  regulate  to  approximately  con- 
stant speed  under  widely  varying  load  conditions.  Another 
advantage  is  found  in  the  fact  that  all  of  the  current  taken  from 
the  line  is  utihzcd  for  power,  the  changes  in  speed  being  obtained, 
not  by  dissipating  a  portion  of  the  effective  energy  in  the  re- 
sistance (as  in  the  case  of  the  armature  resistance  method  of 


690  HAWKINS  ELECTRICITY 

control)  but  by  weakening  the  reverse  voltage  by  inserting 
resistance  in  the  shunt  field  circuit.  Speed  increase  by  shunt  field 
weakening  is  limited,  however,  to  about  10  to  15  per  cent,  above 
the  normal  speed  in  motors  of  standard  construction.  Greater 
ranges  of  speed  can  be  obtained  from  motors  especially  designed 
for  shunt  field  control  but  should  not  be  attempted  with  motors 
of  standard  design  \\'ithout  first  ascertaining  from  the  manu- 
facturer the  maximum  safe  speed. 

Combined  Armature  and  Shunt  Field  Control. — Regu- 
lation by  combined  armature  and  shunt  field  resistance  is  by  far 
the  easiest  way  of  obtaining  a  wnde  range  of  speeds.  Rheostats 
embodying  these  methods  are  known  as  compound  speed  regu- 
lators, one  form  being  shown  in  fig.  762.  Standard  regulators 
can  be  obtained  giving  a  wide  range  of  speed  variation,  and 
special  regulators  may  be  constructed  giN'ing  practically  any 
desired  range. 

Selection  of  Starters  and  Regulators. — Unsatisfactory 
operation  of  these  devices  is,  in  nearl}-  all  cases,  due  to  lack  of 
precaution  in  selecting  the  proper  piece  of  apparatus  for  the  work 
to  be  done.  One  of  the  commonest  errors  is  to  select  a  rheostat 
of  insufficient  capacity.  If  the  current  required  to  operate  the 
motor  at  full  speed  with  no  resistance  in  circuit  be  greater  than 
the  rated  capacity  of  the  rheostat,  overheating  of  the  resistance 
will  result.  An  increase  in  temperature  even  to  a  point  where 
the  hand  cannot  be  held  on  the  enclosing  case  need  cause  no 
apprehension,  but  should  the  resistance  become  red  hot  it 
indicates  that  the  apparatus  is  being  worked  far  beyond  its 
capacity,  and  the  load  on  the  motor  should  be  reduced  or  a 
regulator  of  greater  capacity  substituted. 

If  the  current  required  to  operate  the  motor  at  full  speed  with 
no  resistance  in  circuit  be  less  than  the  rated  capacity  of  the 


OPERATION  OF  MOTORS  091 

rheostat  no  overheating  will  occur,  but  it  will  not  be  possible  to 
secure  the  full  50  per  cent,  speed  reduction  the  rheostat  is  de- 
signed to  give  with  all  resistance  in  circuit. 

In  ordering  a  starter  or  regulator    the  manufacturer  should 
be  furnished  with  the  following  information : 

1.  Horse  power  of  motor  with  which  speed  regulator  will 
be  used; 

2.  Voltage  of  motor; 


""IG.  768. — Various  sizes  of  Watson  commutator.  The  segments  are  punched  from  hard  drawn 
copper  strip  and  are  insulated  from  each  other  and  the  core  by  amber  mica,  of  hardness 
corresponding  to  that  of  the  copper  in  order  that  the  wear  of  mica  and  copper  may  be 
imiform.  The  segments  are  assembled  in  a  ring  under  great  pressure  and  are  repeatedly 
heated  and  tightened,  being  finally  secured  and  rigidly  locked  together. 

3.  Winding  of  motor,   whether  series,   shunt,  or  compound 
wound; 

4.  Nature  of  the  machine  which  motor  is  to  operate; 

5.  Normal  rated  speed  of  motor  to  be  used; 

6.  Maximum  speed  at  which  it  is  desired  to  operate  the  motor ; 

7.  Minimum  speed  at  which  it  is  desired  to  operate  the  motor ; 

8.  Whether  controller  will  ever  be  required  to  reverse  direction 
of  motor  or  to  operate  it  in  one  direction  only ; 


t)92 


HAWKINS  ELECTRICITY 


9.  If  reversible  controller  be  desired,  whether  or  not  full  range 
of  speed  control  is  required  in  both  directions ; 

10.  Whether  the  regulator  shall  be  equipped  with  any  of  the 
following  devices:  no  voltage  release,  overload  release, 
knife  switch,  fuses; 

11.  Whether  button  contacts  or  renewable  contact  segments 
are  preferred; 


A 


Fig.  709. — Organ  blower  speed  regulator;  diagram  showing  operation  and  method  of  installing. 
A  cord  running  from  the  top  of  the  organ  bellows  passes  over  two  pulleys  and  is  then  made 
fast  to  the  chain  furnished  with  the  regulator.  This  chain  passes  around  a  sheave  which 
turns  on  a  post  projecting  from  the  center  of  the  slate  panel.  Attached  to  the  lower  end 
of  the  chain  is  a  weight,  also  furnished  with  the  regulator.  As  the  air  is  exhausted  from 
the  bellows  the  latter  slowly  collapses,  drawing  the  rope  down  with  it,  and  in  so  doing 
turns  the  sheave  from  left  to  right,  thus  cutting  resistance  out  of  circuit  and  increasing 
the  speed  of  the  motor  which  pumps  air  into  the  bellows.  Responding  to  the  inrush  of 
air,  the  bellows  expands,  rela.xing  the  tension  on  the  rope  which  is  now  pulled  in  the  oppo- 
site direction  by  the  weight,  thus  turning  the  sheave  froin  right  to  left,  cutting  resistance 
into  circuit  once  more  and  slowing  down  the  motor.  The  speed  of  the  motor  is  thus 
automatically  regulated  by  the  bellows,  with  the  result  that  a  practically  uniform  pressure 
is  maintained  at  all  times.  In  connection  with  an  organ  blower  regulator  it  is  necessary 
to  install  a  separate  startifig  rheostat.  This  is  required  for  the  reason  that  all  organ 
bellows  leak.  During  the  intermissions  in  the  musical  part  of  the  service,  or  at  other 
times  when  the  blower  is  not  operating,  the  air  gradually  escapes  and  the  bellows  settles 
down,  moving  the  rheostat  arm  to  the  right  and  cutting  out  resistance.  With  the  motor 
at  rest  and  the  bellows  empty  all  the  blower  regulator  resistance  would  be  short  circuited 
and  it  is  therefore  necessary  to  avoid  throwing  the  motor  directly  across  the  line  when 
starting  again.  A  starting  rheostat  with  no  voltage  release  is  suitable  for  this  purpose, 
and  should  be  installed  within  easy  reach  of  the  organist,  so  that  a  moment  or  two 
before  beginning  to  play  he  can  move  the  lever  of  the  starting  box  and  get  the  motor 
into  operation.  Where  remote  control  is  desirable  a  self  starter  can  be  substituted  for 
the  manually  operated  starting  box,  in  which  case  the  entire  installation  can  be  controlled 
by  a  oush  button,  or  single  throw  knife  switch. 


OPERATION  OF  MOTORS 


C03 


12.  Gi\dng,  also,  if  possible,  the  resistance  of  the  shunt  field 
cold,  and  the  shunt  field  cvurent  at  the  maximum  speed 
required.  If  this  cannot  be  ascertained,  give  horse  power, 
voltage,  normal  speed,  maximum  speed  required,  serial 
number  of  motor  and  name  of  manufacturer. 


Fir. 


770. — General  Electric  type  K7  controller  with  cover  open  showing  construction.  The 
niechanism  consists  of  a  long  spindle,  carrying  a  number  of  heavy  brass  or  gun  metal 
segments,  making  contact  for  a  longer  or  shorter  time  with  a  corresponding  number  of 
spring  contacts.  The  spindle  is  provided  at  its  upper  end  with  a  handle,  and  the  various 
contacts  are  made  by  turning  it  through  an  arc  of  about  150°.  For  this  method  a  moderate 
amount  of  resistance  is  employed.  The  first  contact  joins  both  naotors  and  the  full  amount 
of  resistance  in  series  across  the  line,  and  as  the  motors  are  standing  still,  maximum  current 
flows  so  that  they  exert  their  full  torque.  The  moment  they  start  to  revolve,  the  current 
tends  to  fall,  due  to  the  generation  of  a  reverse  voltage;  to  prevent  tnis  and  maintain  a 
heavy  current  for  some  time,  thus  obtaining  rapid  acceleration,  the  resistance  is  arranged 
so  that  it  can  be  gradually  reduced,  until  at  about  the  fourth  notch  the  two  motors  are  in 
series  without  resistance  across  the  line.  To  increase  still  further  the  speed  in  the  above 
type  of  controller,  the  series  fields  may  be  shunted,  and  then  the  ne.xt  steps  place  the  motors 
in  parallel  with  the  resistance. 


Speed  Regulation  of  Traction  Motors. — The  speed  regu- 
lator for  motors  of  this  class  is  called  a  controller,  and  being 
located  in  an  exposed  place  is  enclosed  in  a  metal  casing.  Con- 
trollers are  designed  to  be  used  for  starting,  stopping,  reversing, 
and  regulating  the  speed  of  motors  where  one  or  more  of  these 
operations  have  to  be  frequently  repeated. 


694 


HAWKINS  ELECTRICITY 


The  controller  used  with  a  single  motor  equipment  is  practically  the 
same  as  any  other  single  motor  starting  box,  excepting  that  the  resistance 
has  sufficient  carrying  capacity  to  be  left  in  the  circuit  some  time.  When 
the  motor  is  to  operate  at  full  speed  all  the  resistance  is  cut  out.  To 
reverse,  a  reversing  notch  is  i)laccd  in  the  armature  or  field  circuit,  but 
not  in  both. 

Oues.     What  provision  is  made  to  overcome  the  arc 
when  the  circuit  is  opened? 

Ans.     A  magnetic  field  is  used  with  such  polarity  that  it  blows 
out  the  arc. 


Fig.  771. — Controller  of  the  Rauch  and  Lang  electric  vehicles.  It  is  of  the  flat  radial  type. 
Two  movable  copper  leaf  contacts  of  ample  size  make  all  commutations  necessary  to  obtain 
the  various  speeds.    Five  speeds  forward  and  reverse  are  provided. 


Magnetic  blow  out  coils  are  used  on  all  controllers  designed  for  500 
volt  circuits,  and  on  types  designed  for  lower  voltages  requiring  more 
than  60  amperes  normal  capacity. 

The  coils  are  wound  with  either  copper  wire  or  fiat  strips  of  sufficient 
capacity  to  carry  full  load  current  continuously  without  undue  heating, 
and  after  being  wound  they  are  treated  with  an  insulating  compound 
making  them  moisture  proof. 


OPERATION  OF  MOTORS  695 

Oues.  What  provision  is  made  to  prevent  reversal 
before  bringing  the  controller  lever  to  the  "  off  "  position? 

Ans.  Controllers  having  separate  reversing  cylinders  arc 
fitted  -with  mechanical  interlocks  making  it  necessary  to  place 
lever  in  off  position  before  reversing. 

POINT  RESISTANCE     MOTOR  1  MOTORS 

*R»<»TUSE  FIEU)  »R«*TuaE  ntLO 

3  -w^vr-----^>O^00^3~>CV^ 


4   -V^vvvwj-^v.^-aXQ,^'(XA)        ^"O*^ 

5 


6  -jAwvw^^v^^vp^O^OOO    ^O^TXX) 


i-Y^-^^s^^---\p'<y^wy~''f0^^u3d^ 


8  ^vw^^^w^P<VTXX^^fO^'^00Q   ^ 

g  — J./vw\AAAWvr-*''0^  000      'f'O^OOO 

Figs.  772  to  7S2. — Diagram  of  controller  connections,  illustrating  the  series  parallel  method 
of  two  motor  control. 

Two  Motor  Regulation. — With  a  two  motor  equipment, 
the  controller  becomes  more  complicated  because  it  must  be 
arranged  to  switch  the  motors  in  series  or  in  parallel,  so  as  to 
secure  economy  at  half  and  full  speed.  The  various  connections 
of  series-parallel  regulation  ar^  shown  in  figs.  772  to  782. 


696  HAWKINS  ELECTRICITY 


From  these  diagrams  it  is  seen  that  the  motors  are  first  operated  in 
series  until  all  the  resistance  is  cut  out  by  the  controller  (figs.  772  to  777). 

The  next  point  on  the  controller  puts  the  two  motors  in  parallel  with 
some  resistance  in  the  circuit  (fig. 778), which  resistance  is  gradually  short 
circuited  on  the  remaining  controller  points,  until  at  full  speed  all  the 
resistance  is  cut  out,  the  two  motors  remaining  in  parallel  (figs.  778  to  782). 


Stopping  a  Motor. — If  it  be  desired  to  stop  a  motor,  the 
main  switch  is  opened.  As  the  armature  of  the  motor  con- 
tinues to  operate,  due  to  its  inertia,  it  generates  an  electro- 
motive force  which  sends  a  current  through  the  shunt 
connected  field  circuit  and  helps  to  maintain  the  field  excitation. 
When  the  speed  of  the  motor  has  decreased  sufficiently  so  as  not 
to  endanger  the  motor  should  the  main  switch  be  thrown,  the 
current  in  the  series  magnet  becomes  weakened,  and  the  spring 
throws  back  the  starting  box  arm. 

It  should  be  noted  that  in  stopping  a  motor  having  a  start- 
ing box  provided  with  a  no  voltage  release  simply  open  the 
main  switch  and  do  not  touch  the  lever  because  other\\4se,  the 
self  induced  voltage  of  the  field  circuit  may  puncture  the  field 
winding  or  the  insulation  of  the  adjoining  wires  in  the  starting 
box. 


rKOKRTY  UiKART 
N.  C  State  College 


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ELECTRICAL  GUIDE,  NO.  1 

Containing  the  principles  of   Elementary  Electricity,  Magnetism, 
Induction,  Experiments,  Dynamos,  Electric  Machinery. 

ELECTRICAL  GUIDE,  NO.  2 

L  ■  The    construction    of    Dynamos,    Motors,    Armatures,    Armature 

f '  Windings,  Installing  of  Dynamos. 

ELECTRICAL  GUIDE,  NO.  3 

Electrical  Instruments,  Testing,  Practical  Alanagement  of  Dynamos 
and  Motors. 

ELECTRICAL  GUIDE,  NO.  4 

.  Distribution  Systems,  Wiring,    Wiring  Diagrams,  Sign  Flashers, 

Storage  Batteries. 

ELECTRICAL  GUIDE,  NO.  5 

'  Principles  of  Alternating  Currents  and  Alternators. 

ELECTRICAL  GUIDE,  NO.  6 

Alternating  Current  Motors,  Transformers,  Converters,  Rectifiers. 

ELECTRICAL  GUIDE,  NO.  7 

Alternating    Current   Systems,   Circuit  Breakers,   Measuring   In- 
struments. 

ELECTRICAL  GUIDE,  NO.  8 

Alternating    Current    Switch    Boards,    Wiring,    Power    Stations, 
Installation  and  Operation, 

ELECTRICAL  GUIDE,  NO.  9 

Telephone,  Telegraph,  Wireless,  Bells,  Lighting,  Railways. 

ELECTRICAL  GUIDE,  NO.  10 

Modern  Practical  Applications  of  Electricity  and  Ready  Refer- 
ence Index  of  the  lo  Numbers. 

Theo.  Audel  &  Co.,  Publishers.      "  ^n'^yor™"'^' 


X' 


